Art. II.—Some Aspects of the Terrace-development in the Valleys of the Canterbury Rivers.
[Read before the Philosophical Institute of Canterbury, 1st May, 1907]
The substance of this paper formed part of an ex-presidential address delivered before the Philosophical Institute of Canterbury. Considerable alterations and additions have been made to it, but the main conclusions stated originally have been retained, and further evidence put forward in support of them. The paper attempts to give, first of all, some account of the mode of formation of the terraces in the main river-valleys, and then considers the evidence of elevation and depression of the land during late geological times. Without attempting to summaries and criticize all that has been written on the subject, the author gives some account of this, especially in its bearing on terrace formation and finally he draws attention to the importance of formation, and finally he draws attention to the importance of frost erosion in the Canterbury mountains, and suggests that the supply of waste is a powerful factor affecting the erosive power of the rivers, and therefore, directly or indirectly, the conditions favourable to terrace-development.
The rivers of Canterbury which will be considered in this paper are those of the middle district—viz., the Waimakariri, Rakaia, Ashburton, and Rangitata. They closely resemble each other as regards the conditions under which the valleys were formed, with the partial exception of the Ashburton, so that statements made about one generally apply to all. They all rise in the main range of the Southern Alps or close to it, and flow in a south-easterly direction till they reach the sea, the first half of their course being through the mountainous region of western Canterbury, and the second half across the plains which fringe this region on the south-east. The rocks of the first portion consist principally of folded slates, sandstones, greywackes, and allied sedimentaries chiefly of Lower Mesozoic age. Palæozoic rocks doubtless occur on the eastern and western margins of the mountain region, but the general absence of fossil evidence renders their true age difficult to determine.
The folding of these rocks occurred most probably in Upper Jurassic times, but traces of an earlier folding are also found. They are distinguished throughout the whole area by excessive jointing, which has rendered them particularly susceptible to the disintegrating action of frost, and has caused them to split readily into more or less rectangular and prismatic blocks. This effect is so marked that many of the mountains are, for several thousand feet in altitude, covered with a coating of déris so thoroughly that solid rock is scarcely visible. This is constantly moving down to lower levels under the influence of the transporting agents which operate in mountain tracts, but principally owing to the torrents formed by melting snows.
The rocks of which the plains have been formed consist chiefly of gravels, more or less perfectly rounded, and of sands, silts, and mud. The last predominates in the outer margin of the plains. There is in some cases an admixture of volcanic material and limestone, but these are of relatively minor importance.
The western mountain area formed at one time part of a great peneplain, and this has now been thoroughly dissected. The paths of the rivers are generally at right angles to the strike of the beds, so that the main streams may be called consequent, while the tributaries are generally parallel to the strike, and are therefore subsequent; but, owing to the age of the river-valleys and the influence of other disturbing agencies, marked departures from this rule frequently occur. A recent severe glaciation, after the valleys had reached a mature stage, exerted great influence on them, and its effects are still plainly evident. The rivers are all perfectly graded at the present time, but it is highly
likely that they had reached an approximately similar condition in Oligocene times, as pointed out by Captain Hutton.
Although the mountain tract of the province has been thoroughly dissected, the plains are practically undissected, if we omit consideration of that dissection which is due immediately to the rivers themselves. They receive hardly any tributaries after they leave the mountains; the rain which falls on the plains soaks rapidly through the porous beds, and finds its way to the sea by percolation through the underlying shingle. The rivers do receive some tributaries—e.g., the Kowhai runs into the Waimakariri, and four rivers coalesce to form the Ashburton—but they all rise in the foothills, and derive little of their water from the rainfall on the plains. It is therefore evident that there is a marked contrast between the physiographic conditions of the upper portion of the rivers and that of their lower courses, and hence the conditions which affect the terrace-development are highly dissimilar.
If we examine the valleys of the large rivers we find that their courses may be divided into four parts, relative to their terrace-development: (1.) The torrent path, where terraces are, as a rule, absent. (2.) A wider valley path, where the rivers are aggrading their beds, river terraces being absent, but glacial terraces or shelves common. (3.) A gorge path, where rivers burst through the outer range of Palæozoic rocks on a line running through Mount Hutt and Mount Torlesse: in this case the terraces have their highest development. (4.) A plain path—i.e., the path from the foot the mountains to the sea, where terraces are again strongly developed, but are, as a general rule, of a simple and continuous character.
The Torrent Path.
The rivers begin as fair-sized streams from the terminals of glaciers, and this part of their course shows the general characters of torrent and glacial erosion. The valleys are typically U-shaped, with flat floors and sides so steep as to be at times unscalable for miles. They show signs of having been recently swept clean, but are filling again with waste coming in from the sides. There are no terraces except those due directly to glacier action. Lateral moraines occasionally form terraces, but only in those places where they have been protected from the scouring action of the wild torrents which sweep this portion of their valleys. A frequent position for these terraces is round the end of a spur, and they slope down the valley at a steep angle, indicating a rapid fall in the level of the surface of the glacier, owing to its expanding as it accommodated itself to a part of the valley where the cross section was greater. The
valley-walls also show signs of the truncation or partial truncation of the spurs. This is often attended by the formation of short glacier shelves due to the erosive action of the glacier. These shelves occur particularly where the glacier came over the shoulder of a spur and cuts down its bed in a manner analogous to the action of a corroding stream. Good illustrations of this are to be seen towards the head of the Waimakariri, up the Bealey River, at Arthur's Pass and in the neighbourhood of the West Coast Road between the Cass and the head of Sloven's Creek; but these last cases belong to another part of the river-valley.
The Valley Path.
The first part of the river-course grades into the second. Here the valley is flatter and wider, and still shows signs of glacier erosion; glacial terraces of shelves are common in much the same position as in the first part of the river-course, frequently in sets of three, as noted by Captain Hutton. In some cases it seems likely that terraces are formed by the erosive action of tributary glaciers. These are turned round by the resistance of the main glaciers at the junction, and made to override the projecting spurs on the downstream side of the valley. The spurs are thus cut down to a marked degree, and show true terraces of primary erosion. These terraces are cut out of solid rock, and have a steep fall downstream—steeper than the grade of the valley, and of no great length parallel to its axis. This action is most probably going on now where the Ball Glacier joins the Tasman; and if we could see the side of the valley underneath, it would almost certainly show these glacier terraces. Good illustrations occur where a large stream, the name of which is unknown to me, joins the Waimakariri on its south bank about six miles above Bealey. This case is a most important one, as it shows conclusively that even the smaller tributary valleys were formed previous to the recent glaciation. The stream enters the main river by a channel cut out of the solid rock, and in the bottom of this glacial striæ are plainly visible, running across the bed of the stream and nearly parallel to the axis of the main valley. The channel must have been eroded previous to the glaciation, as it is very well marked, and depressed about 50 ft. below the level of the surrounding rocks, which are remarkably ice-worn as well, and form part of a truncated spur entering the main valley at right angles. It appears almost impossible that the channel of the stream can have been formed solely by glacier erosion, and the recency of the glaciation is emphasized by the perfection of the markings in a position where they are very likely to be the effaced.
Apart from the glacier shelves there are no terraces, as in this portion of their course the rivers are aggrading their beds. The supply of waste is almost inexhaustible. It is poured in by every tributary stream and every shingle-slip, and the grade of the river is not sufficient for its transportation. Where the tributary are large, the result is to flatten the grade of the main river above the junction and to push the main river over to the opposite side of the valley. This effect is especially marked in the case of the Bealey River. Surveys carried out by Mr. Edward Dobson, C.E., when searching for the best route to the West Coast, show undoubtedly that the grade of the Waimakariri has been considerably modified, in the manner suggested, by the action of this large tributary. The main river is not competent to remove the load poured into it.
This portion of the river-valley has been deepened by glacier erosion, though not to any great extent, as the roches moutonnees in the Rangitata, Rakaia, and Waimakariri valleys show; but the rivers have no power now to form terraces, except very low and temporary ones.
The valleys at the head of Lakes Pukaki and Tekapo, in the basin of the Waitaki, show the conditions which prevailed in all the valleys in Canterbury after the maximum glaciation was past. A lake occupied the Lower Rakaia Valley, ponded back by a bar stretching across the mouth of the gorge; a similar lake filled the Waimakariri Valley from the gorge as far as the junction with the Hawdon River, if not farther, and in all probability one existed in the Rangitata.
The formation of these lakes is due to one of two causes—(1) to the elevation of the land along an axis which coincides with the outer range forming the eastern boundary of the Southern Alps; or (2) to glacier erosion.
If this axis of elevation really exist, it would be approximately in a line with that running through the Kaikoura Mountains, where crustal movements are now going on. This axis has, without doubt, extended from the Kaikouras in a south-westerly direction, and perhaps the great Waipara fault has been associated with this earth-movement. The fault is of very recent date, and coincides with the gorge of the Waipara River, and has a down-throw to the north of over 1,000 ft. Unless this fault is due to lateral movement, it is necessary that a thickness of 1,000 ft. has been removed form rocks about the Weka Pass and Waipara River, for the escarpment of the Mount Brown beds presents a tolerably even line both north and south of the fault-line. The physical features are more easily explained by a lateral movement of the rocks, resulting in fracture along the gorge of the Waipara. The force producing this rupture must have come
from the south-east, and it is therefore likely that it affected the rocks further south-west. If this axis extends into mid-Canterbury, it might account for the slight break in the grade of the rivers which occurs at their gorges. They have a flat grade above and a steeper grade below, as the following table taken from Haast's “Geology of Canterbury” will show:—
|Rangitata—||Distance in Miles.||Fall of Rivers, per Mile.|
|From junction of Havelock and Clyde to beginning of plains||29||35|
|From beginning of plains to railway||23 ¼||39 ¾|
|From railway-crossing to sea||8 ¾||29|
|From Clearwater Creek to beginning of plains near Two Brothers||11½||37½|
|From Two Brothers to railway||25||44|
|From railway to sea||10½||28¾|
|From junction of Wilberforce River to Gorge Island||19||25½|
|From Gorge Island to railway||21½||23½|
|From railway to sea||16||23¼|
|From junction of Bealey River to junction of the Esk River||21||24|
|From junction of Esk River to junction of Kowhai River||17||33|
|From junction of Kowhai to White's old accommodation-house at height of 605 ft.||15||26¼|
|From White's accommodation-house to tidal boundary||22||27½|
The flat grade followed by a steeper grade is apparent in the Rangitata, Ashburton, and Waimakariri; but in the case of the Rakaia there is no marked break: its bed is almost uniformly graded for a long part of its course. This difference in grade may be due to crustal movements, but I think it is more probably due to glacier erosion; however, there is no impossibility that it may be due to both causes. The Canterbury valleys are of very ancient date, and were developed to a mature stage before the recent glacier extension. The glaciers hollowed out their middle portion, particularly where two valleys join, but left across the mouth a bar of solid rock, owing to the falling-off in erosive power near the terminal face. Behind this bar there is always a deep depression or basin in the solid rock—
as much as 500 ft. in the case of the Rakaia—and the hypothesis of the axis of elevation seems hardly competent to explain this remarkable occurrence, in the valleys of all the principal rivers.
The deepened portion of the valley has been filled with glacial silt and angular débris from the hills; traces of old sublacustrine fans or deltas are to be seen in many places. These sedimentary beds are now being eroded by the rivers as they cut down through the bars of solid rock that form the main floor of the gorges by which the rivers issue on to the plains. The present shape of the river-valleys is due, therefore, to the modifying action of glaciers and other agencies on a previous matured stream system, the rough features of which were antecedent to the glacier extension. With this explanation it will be possible to consider the third division of the rivers' course as regards their terrace-development.
The Gorge Path.
In this paper I apply the term “gorge path” to that part of the river-course form its first meeting the lacustrine beds above the bar of rock till it has freed itself from all rock obstructions in the upper portion of the plains. It is only the middle section of this which shows the true character of a river gorge; but it is most convenient to consider the more extended length with regard to the terraces.
The three principal rivers of northern Canterbury burst through the outer range of mountains by gorges of a similar type. The Ashburton Gorge was formed under peculiar conditions, owing to the great changes in the directions of drainage caused by the extension of the glaciers. If we take the Rakaia as a typical case, we have a river flowing through a bed of glacial silt which partially filled the old Rakaia Lake, and then coming to a winding gorge cut out of the solid rock which forms the floor of a wide valley. This valley is nearly three miles in width, tolerably flat, but covered with heaps of morainic and fluviomorainic matter. The river flows in meanders at a depth of nearly 500 ft. below the main floor. This winding trench was begun immediately after the ice began to retreat, no doubt while the lake was in existence above the solid bar of rock. Owing to increased power of corrasion, the river has deepened its meanders far below the upper floor of the gorge, and is now actively removing the projecting spurs between them. Several cases of nearly demolished spurs and of islets in the river-bed which are now quite cut off are to be seen in the Waimakariri as well as in the Rakaia.
Overlying the outer portion of the flat valley are the gravels of the Canterbury Plains. They rise in the Waimakariri fully 200 ft. above the wide floor of the upper gorge, and are found at places in the gorge itself. The plains are formed by the overlapping and coalescing of the fans of great Pleistocene and Post-Pleistocene rivers, and have covered up nearly all irregularities in the solid floor of the land; though at such places as Gorge Hill, Burnt Hill, View Hill, the older rocks are visible above the level of the plain. Owing to the rivers cutting down through the gravels the solid floor has been exposed in other cases. In this gorge portion the terrace-development is most perfect. In the Rakaia, sixteen terraces may be counted from the top of the heaps of morainic matter down to the level of the river—that is, in a height of 500 ft. The other river-valleys show similar phenomena, though perhaps the sequence is not so complete.
It seems highly likely that this portion of the river-valley was filled by gravels up to a certain level previous to the glacier maximum, as the moraines and fluvio-morainic deposits overlie the gravels at the mouth of the gorge. This filling-up might have happened several times during the Tertiary era, as our valleys were largely excavated before the Oligocene period, as emphasized by Captain Hutton, and it is possible for a glacier to override even loose gravels at its terminal face without displacing them. Some of the lower gravel-beds just below the Rakaia Gorge are so highly coloured by hydrated iron-oxide as to afford an easy means of distinguishing them form the upper gravel-beds. This points to a considerable lapse of time in order to allow for this oxidation, and suggests a much older date for the lower gravels. However, this evidence is by no means conclusive. The fact that the glacier deposits overlie the gravels of the plain is important, as showing that the extension of the glaciers was subsequent to the deposition of the gravels in this upper portion of the plains.
Terraces near the Gorge.
An examination shows that a great majority of the terraces in this part of the river's course are connected in some way with obstruction met with by the river as it cut its bed through the lacustrine silt just above the gorge proper, or through the gravels of the plains just below it. A number in the gorge itself are rock-cut terraces covered with a thin veneer of gravel.
As nearly all terraces are the remains of former flood plains—whether they are cut terraces or built terraces, or formed by a combination of both processes—any circumstances which tend to preserve these flood plains will favour terrace-development.
I select the Rakaia Gorge as typical of all the rivers, because it is the simplest in form, and now consider how the terraces arise here in the light of the fact that they are the remains of former flood plains.
In the Rakaia Gorge they are nearly all connected with obstructions:—
(1.) The highest ones are intimately related with the morainic heaps of the old glaciers, or those heaps of morainic material but roughly assorted by fluvio-glacial action. The rough angular and subangular blocks were rather difficult to remove by river action, and they protected portions of the original gravels, or they allowed flood plains to be built up under their protection either on the upstream side or on the downstream side of the obstruction. The topmost terraces are nearly all associated with these morainic heaps, and they form a series totally distinct from the lower ones.
(2.) The lower series of terraces have in most cases some connection with the underlying hard rocks, which in the Rakaia Gorge are principally volcanic. The flood-plain remnants are frequently on the downstream side of prominent bluffs of solid rock. These protect flood plains which have been built up on a foundation of solid rock or cut out of former river gravels. The bluff causes the stream to move across towards the opposite bank. Flood plains are therefore likely to form under its protection, as there is likely to be relatively slack water immediately below it in which suspended matter is dropped. A flood plain is thus rapidly formed, and when formed the bluff continues to protect it, prolong its life, and thus promote terrace-development. If these terraces have been formed on a flood of solid rock they will be doubly secure, owing to the influence of cause No. 3, mentioned subsequently. If, however, they are terraces cut out of old gravels, the bluff will still exert a protective influence. The former condition explains the occurrence of most of the terraces in the gorge proper; immediately after the river has passed through the gorge, the latter is the most important. The sheltering action of bluffs is very apparent in the Ashburton and Waimakariri Valleys.
(3.) The third condition which promotes terrace-development near the gorge of the river is the occurrence of defending ledges of solid rock, which the river exposes as it lowers its bed through the gravels and lacustrine silts above the rock bar, or through the gravels of the plains immediately below it. The influence of defensive ledges was urged by Hugh Miller the Younger in a paper read before the Royal Society of Edinburgh in the year 1882. I have not been able to see this paper, but an account of Miller's theory was published in the “American Journal of
Science,” vol. xii, 1902, by Professor W.M. Davis when describing the Terraces of Westfield River, Mass. The phenomena he there describes are reproduced in our rivers: good examples occur in the Rakaia, but excellent ones are to be seen in the Waimakariri at Little Gorge Hill, where the railway crosses.
It may be urged that there is no great difference between cause (2) and cause (3). It is quite true that a defending ledge may, under certain circumstances, become a protecting bluff; but the latter will be after the bluff has done duty as a defending ledge and the river has lowered its bed considerably. However, in very many cases the action is quite distinct, and some protecting bluffs have never been defending ledges.
(4.) The same result is obtained also by the defending action of a tributary stream which pours in a load of sediment. According to the general law of stream action, a tributary if fully supplied with waste will deposit it on joining the main river flowing on a gentler grade. In any case, the tributray pushes the main stream over. This action is much the same as a defensive ledge. The bank is defended from the destructive action of the main stream by the force of the tributary. If the main river can lower its bed, then we shall expect to have a series of terraces; but they are different in character from those due to the previous causes. They are usually lower and broader, and the sequence is perfect; they are extremely common, and seen in almost every case when one stream joins another. They afford the most complete record of the oscillations of a river across its bed, and are more remarkable in this respect than those due to cause (3). Splendid examples of such terraces are to be seen at the junction of the Kowhai with the Waimakariri, and also at the junction of Woolshed creek with the South Ashburtron.
Closely connected with the action of tributary streams is that of talus cones. One of the causes of the partial destruction of the terraces is the formation of talus cones from the high shingle banks. These grow, owing to the erosion towards the head of the cone, till intermittent streams flow down them. Erosion then proceeds apace. In this way a portion of the terrace is rapidly destroyed; but the cone or fan on the floor of the river-valley protects the remaining portion of the terrace from the erosive action of the river, so that rapid destruction of one portion prolongs the life of the remainder. This action is to be seen in many places near the Rakaia Gorge.
Cases of all these four modes of terrace-development are to be seen in the gorge itself, or immediately after the river debouches on the plains. In the Rakaia they may be seen as far down as the Curiosity Shop beds, about three miles
below the Gorge Bridge, where harder sandstones and limestones underlie the shingle. Here there is a good example of the combined effects of the above-mentioned agencies. A protecting bluff determines the commencement of a terrace on its down stream side, and also protects the bank so as to cause one on its upstream side. The bluff also acts as a defensive ledge to the higher terraces, which were at first dependent on the larger and more resistent blocks of the terminal moraine of the Old Rakaia glacier. A little above the bluff are excellent examples of the protective action of talus cones, and on the opposite side of the river, a little higher up, of the action of a tributary stream developed from a talus cone.
All the above-mentioned causes are in operation in the Ashburton and Waimakariri Rivers, but it must be noted that all terrace remnants cannot be assigned to them. A number of smaller remnants are not related in any way to obstructions—that is, as far as can be detected at present. It is possible that some of the terraces above the gorge, where the river is cutting out the silt and gravels filling the old lake, may be the remains of old lacustrine beaches.
The Plain Path.
The fourth division of the river-course is that across the plains, when the river no longer meets solid obstructions in its bed. The terraces here are simple and continuous in character; the sequence is not so complete, as the remains of flood plains are, as a rule, fewer and higher. The river-bank sometimes drops from the level of the plain to that of the water, a distance of as much as 400 ft. in a single face. These terraces are caused by the river moving across its bed lowering its channel as it does so, making and again destroying its flood plains. One reason why the terraces are so high is that the lower ones, being composed of loose and incoherent materials, are readily removed, and the river is able to swing, in some cases, the whole width of its former highest channel. The high terraces are formed by the river planing off a strip every time it swings across its bed, and swinging to the full width possible a large proportion of the times. There is thus a tendency to produce high and simple terraces. These are higher, however, in the upper part of the plains, and get lower as the river approaches the sea; in fact, it is certain that the Waimakariri is rapidly raising its bed in its lower portion—so much so that it threatens danger to Christchurch, and demands the erection of costly protective works as a defence in flood-time. On one occasion, in the year 1868, the river burst through, flooded the neighbouring country, took a course by an old river-bed, and ran in a considerable
body of water right through the city. The danger of this recurring is all the more serious as the river is now showing a decided tendency to wear away its southern bank. The Rangitata and Rakaia are also aggrading their lower portions in all probability. Mr. Edward Dobson informs me—and examination of the railway-levels confirms his statement—that the river is running at a higher level at the Rakaia railway-bridge than its old bed immediately north of it. The old bed is about three miles and a half wide, and bounded on the north by a high terrace, which the railway descends near the Bankside Station. At the foot of this terrace the bed is 317 ft. above sea-level; but at about two miles and a half from it, going south, the level of the bed rises to 339 ft., and falls in the next mile to 337 ft., which is the level of the water at the bridge. This last height is subject to slight variation, depending on the position and size of the anastomosing streams. These facts seem to show that in former times the river ran 20 ft. below its present level, and that in all probability it is now filling up the broad, flat trench which it once eroded out of the tolerably level plains. It is thus showing the characters of a stream on a fan, though in this case the fan is confined by the old river terraces.
The section across the Rakaia river-bed at the railway is most instructive. The railway surveys show that, in most-case, the low terraces within the main terrace are not absolutely flat, but have a slight inclination away from the river, being higher at the edge than they are some distance back. They thus exhibit a form which resembles in some degree the scarp slope and dip slope of sedimentary rock. The scarp corresponds to the trerrace, and the dip slope to the gently backward-falling surface of the terrace. This resemblance is merely one of form, and not of structure, and it is exactly what might have been expected in a case where terraces are partly due to erosion, and partly to building up a flood plain, the latter process being the more important. It is unfortunate that this interesting section cannot be reproduced.
From the foregoing description of the mode of occurrence of the terraces, it is evident that there must be some cause or causes of exceptional character which have contributed to their formation. In order that a river may form terraces on the scale that occurs here, it must have considerable power of corrasion, both vertical and lateral, and in order to form high terraces the former must be relatively more important. I will therefore consider the circumstances that affect the power of
corrasion, and discuss briefly their bearing on the case in question.
The three main circumstances which affect the corrasive power of a river are—(1) its gradient, (2) its volume, and (3) its load.
The following table, taken from Haast's “Geology of Canterbury,” gives the grade of the rivers on that part of their course between their gorges and the sea. Alongside this, for purposes of comparison, I have also put the grade of the plains where the rivers cross them:—
|Slope of Rivers in Feet per Mile.||Slope of Plains in Feet per Mile.|
These figures give the average slope, but in both the grade of the rivers and also in that of the plains there is a perceptible flattening on approaching the sea.
The following features are shown by this table:—
(1) The rivers all have a steep slope as they cross the plains—in fact, they are still mountain torrents. They should therefore be eroding their beds very rapidly, as their banks are composed of incoherent material, were the erosive power given by their high grade not partly counteracted by other influences. Owing also to this lack of consolidation, lateral corrasion is relatively great. In the upper portion of the plain track, vertical corrasion is more important, so that the terraces are higher; but in the lower part, lateral corrasion becomes more important, and the terraces are much broader and lower.
(2.) The slope of the bed is dependent on the size of the river. The smallest river has the most rapid fall per mile, and the largest river—the Rakaia—the least fall.
(3.) In every case the slope of the plain is greater than the slope of the river, but there is no connection between the slope of the plains and the size of the present river crossing them.
Changes in the Height of the Land.
As the grade of the rivers will depend either directly or indirectly on the height of the land above sea-level and its distance form the sea, it is necessary here to consider the evidence for elevation and depression.
(1.)Evidence for a Recnet Elevation
The existence of peat-beds, as well as buried logs—that is, an old land surface—is proved by artesian-well borings in the Christchurch area. Peat has been found at the following depths: at Ward's brewery, 400 ft.; at Sydenham, 500 ft.; and at Islington, 700 ft., below the surface of the ground. As the first two places are less than 20 ft. and the last only 112 ft. above sea-level, the evidence is convincing that the land stood at least 600 ft. higher than at present when the outskirts of the plains were formed. This proves a substantial elevation in recent geological times; and as artesian borings are put down further a greater elevation may be proved, as only in the immediate neighbourhood of the Port Hills has solid rock been reached by such borings.
Additional positive evidence of an increased height of the land is to be found in the bays which surround Banks Peninsula. They are all, or nearly all, drowned valleys, and were formed when the land was higher. In most cases they are valleys which have been formed wholly by water action. In cases of Akaroa and Lyttelton Harbours, the original craters of volcanoes have, perhaps, been enlarged by explosions, but certainly have been further amplified by water erosion and extended into the valley form. The exposed floors of these valleys grade into the submerged portion. The usual depth of the bay near its outlet to the ocean is from 6 to 8 fathoms—that is, from 40 ft. to 50 ft.—and this gives the minimum elevation necessary to allow the valleys to be formed. But all the bays have been filled to a marked extent by mud washed from the hillsides, so that no accurate estimate can be made of the depth of the rock floor beneath. Borings in search of artesian water-supply put down in the valley behind summer failed to reach either water or solid rock at a depth of 200 ft.
The date of this elevation is difficult to determine in the absence of any fossil evidence or any other accurate time indication; but, taken in conjunction with the evidence from artesian wells, it is, I think, of fairly recent date. Another proof that the land has recently been higher is afforded by the shape and position of the valleys of the streams near Timaru. In most cases they are submerged where they enter the sea.
The evidence from the valleys as well as that from the wells proves conclusively that the land was recently much higher, certainly as much as 600 ft. This elevation would produce a great extension of the land eastward, as the sea-bottom is sensibly flat till the hundred-fathom line is reached at a distance of about forty miles from the present coast-line. Then the depth increases very rapidly to over 1,000 fathoms within the next
few miles. This submarine bank or shelf no doubt marks the utmost eastward extension of the land since Pliocene times. The fan of the Rakaia and Ashburton at one time stretched further east than the present coast-line, as pointed out by Sir Julius von Haast. This would probably have been so extended during a period when the land was at a higher level. On depression setting in, the outer segment of the fan was swept away owing to its being exposed to the full force of the heavy seas and the strong northerly drift on the coast; and this would, no doubt, contain that portion where the streams were actively aggrading their beds. In the case of the Waimakariri, however, this portion has only been submerged, not actively eroded, owing to the protection afforded by the volcanic mass of Banks Peninsula and its submarine easterly and north-easterly extension. Soundings marked on charts show this extension, and also show that the depth increases very gradually form the mouth of the Waimakariri for some distance out into Pegasus Bay. The coast-line here is not marked by any cliff such as occurs on that part of the Ninety-mile Beach near the mouth of the Ashburton River and on the coast near Oamaru. In this place erosion of the coast-line by the action of the waves is extremely rapid, and threatens serious loss in the near future unless adequate protection is given.
An elevation of even 600 ft. would have considerable effect on the climate of the country. In the first place, it would largely increase the extent of country above the snow-line, and hence cause a great extension of the glaciers. The present terminal face of the Tasman Glacier is 2,460 ft. above sea-level; in increased height of the land of, say, 600 ft. would bring it down nearly to the upper end of Lake Pukaki, which is 1,550 ft. above sea -level—that is, supposing the glacier would reach the same distance above sea-level in time of high land as of low land. This supposition may not be strictly accurate, as it is quite possible that the glacier would come down further owing to the increased accumulations of snow; but even if not, the effect of the elevation would still be very marked.
The effect of high land is easily seen on comparing the size of the glaciers at the head of the Waimakariri and Rakaia with those near Mount cook. Even allowing for the increased average height of the peaks in the last-named locality, the glaciers are of enormously greater importance and come down to a much lower level. The height of the terminal face of the Tasman Glacier is 2,460 ft., while that of the Lyell Glacier at the head of the Rakaia is 3,568 ft., and that of the Waimakariri 4,162 ft. above sea-level.
It is possible, therefore, that, owing to increased snowfall
due to elevation and to larger collecting-grounds, the proved elevation of 600 ft. would cause a marked glacier extension; it might even cause a glacier epoch. Captain Hutton has previously explained the advance of the glaciers as due to increased height of the land, and pointed out, form biological evidence, that there could have been no marked refrigeration of the climate.
Another effect of elevation of the land would be to cause desert or steppe conditions to prevail on the plains at the foot of the mountains. Owing to their increased height, the mountains would intercept more of the moisture brought by prevailing westerly winds from the Tasman Sea, which, owing to its depth, probably existed under the same conditions then as now. The mountains would then intercept more moisture, and cause it to fall as snow on the higher levels. Their eastern slopes near Mount Hutt and Mount Torlesse receive their chief rainfall from the west; but, when the higher central ranges cut off this moisture, the eastern slopes would receive much less of this westerly rain. Further, owing to the coast-line being so far to the east of its present position, there would be on the plains a smaller rainfall from the prevailing winds which at present supply the coastal lands. Even at the present time the plains of Canterbury experience a modified steppe climate. The average rainfall for Hokitika is about 119 in. per year, while at Lincoln, near the eastern border of the Canterbury Plains, it is only 25in., and in one year it fell as low as 14 in. The average annual rainfall for Christchurch is only 23 in. These steppe conditions would be intensified during a period of elevation, and the climate would resemble that of Patagonia or Thibet as it is at present.
The present steppe conditions are marked by the great number of xerophilous plant as which are found in Canterbury and Otago, and there are indications from their life history that the desert conditions were at one time more severe.
In his admirable paper on the “Plant Geography of the Waimakariri,” Dr. Cockayne draws special attention to the present climatic conditions of the country, and emphasizes the existence of steppe conditions. When speaking of the eastern climatic plant-region he says, “The Œcological condition of this region is essentially xerophilous. This is not to be wondered at when the small rainfall and constant drying winds in conjunction with the usually stony soil is considered.” In this same paper, in giving expression to an opinion of Diels, the great Œcological and systematic botanist, he further says, “Diels was much struck with the extreme xerophilous character of many plants, which he considered out of all proportion to any severity of climate they have now to endure. At the present
time the driest regions of New Zealand are less arid and possess a more equable climate than Middle Europe, so he considered Carmichaelia, Hymenanthera, Corokia, and some others to be descendants of a forest flora which had been forced to retreat northwards during a rising of the land, which led to the formation of a dry easterly steppe region, where survivors of the forest had become modified and assumed the structure and physiognomy of desert plants.” If this opinion of Diels is correct, I think the conditions are easily explained by an increased height of the mountains modifying the climate. However, Dr. Cockayne shows in his paper that the present conditions are severe enough to account for the plant modification.*
(2.)Evidences of Depression.
The evidence for the lowering of the land below its present level is as follows:—
(1.) Marine terraces occur at Kaikoura, Port Robinson, Amuri Bluff, Motonau, Conway River, and at Banks Peninsula. They are found as high as 600 ft. above present sea-level at Amuri Bluff. The first five of these have been recorded previously by Haast, Hutton, Hector, and McKay, but the last case has not been previously noted as far as I am aware. The evidence for this is as follows: Round the coast of Banks Peninsula the headlands have in many cases flat extremities. The lava-flows which form them dip outwards at low angles, but the edges of the streams are truncated and cut level on the upper surface. The greatest height at which I have noted this marine terrace is at Lyttelton heads, where the elevation is over 450 ft.; the same phenomenon can be seen at Whitewash Head, near Sumner, and at the Long Lookout Point. It is well marked, besides, in other places. The height of this terrace diminishes, as a rule, on those parts of the coast-line which would be exposed during submergence to strong currents and heavy seas. It is low on the southern side of the peninsula. I have not come across in any place traces of marine organisms, but it is not likely they would occur plentifully, or be preserved when they did occur, in such a position. One of the principal conditions which promote rapid erosion on rocky coasts seems to be the presence of strong currents, which can sweep away the material dislodged by wave and other action. Headlands which stretch out far into the sea, particularly if the water be deep on either side, will therefore commonly show a marked wave-cut
[Footnote] * Dr. Cockayne has told me privately that he has latterly modified his opinion somewhat, and now thinks that present conditions are hardly severe enough to account for the xerophilous plant forms.—R. S.
terrace, while an even coast-line will show none. Thus we have the remarkable shore platforms at Kaikoura Peninsula, but hardly any sign of them on the steep hills to the north and south. The conditions would be extremely favourable for the cutting of distinct shore platforms on the spurs of Banks Peninsula during a period of depression.
(2.) The existence of the silt deposit or loess was held by Captain Hutton to be a proof of subsidence. If it is a marine deposit, it undoubtedly proves that the land was much lower—quite 1,000 ft.—as may be inferred from the distribution of the deposit, and its present occurrence so far above sea-level would be a proof of subsequent elevation. However, there are strong reasons for believing it is a wind deposit, and I know from conversations with Captain Hutton that he was not quite satisfied with some of his evidence. One difficulty which strikes me with regard to Captain Hutton's contention is the following: The so-called silt must have been formed of glacial rock-flour during a period of sever glaciation—i.e., during a period of marked elevation of the land. All observers are agreed, I believe, in this. Now, Captain Hutton's theory demands that it should have been distributed into its present position by marine action during a time of depression of the land. It is absolutely impossible that the two processes could have gone on simultaneously in the Canterbury area. If the silt were swept down by great rivers issuing from the glaciers and distributed by the sea at their mouths, the area of deposition would be forty or fifty miles to the eastward of the present coast-lines. Further, if the sea advanced to cover the Canterbury Plains, the glaciers would then have disappeared, or have lingered on only the very highest parts of the Southern Alps. The sea must therefore have distributed the silt during a time of depression posterior to the time of elevation when glaciation was at its maximum. It would have been expected that the silt would be thickest in the hollows and on lower ground. Such in not the case, however; it shows a marked tendency to be thickest on the spurs and to thin out on low ground. In this way it closely resembles the distribution of the loess in the Valley of the Mississippi, to explain which the aid of the sea has never been called in.
Professor A. heim, of the University of Zürich, an observer of wide experience, and an authority of the greatest weight on glacial and allied problems, differed with Captain Hutton on this point. After a visit to New Zealand he published in Zürich, in the year 1905, a paper which has many valuable observations on geological problems in this country. The following is a translation of his remarks in this work on our so-called less:
“When the great glaciers which were thrust forward to the outlets of the alpine valleys receded, and the ground moraines which were left behind were dried up by the north-west wind (Föhn), then the fine dust was blown far over the surface right up to the sea. The deposit of dust accumulated in the form of the fertile loess. The, as we see in many parts of Germany, the loess covered the land-surface, sometimes from half a metre to a metre in thickness, and sometimes form 10 to 15 metres. Where it breaks away on the upper edge of the river-bed region it forms perpendicular walls, and here long-buried moa-bones frequently appear. But even now the loess formation is going on. We have ourselves-seen how thick are the clouds of dust whirled up form the broad, shingly river-beds by the north-west wind and spread over the cultivated land. The rain, when it falls afterwards. unites the dust with the agricultural land. A part of the fertility of the eastern plains depends on the loess covering.”
After a general consideration of the evidence, and from my own observations, I have come to the conclusion that the loess has not been beneath the sea. It is very thick on the hills between Tai Tapu and Birdling's Flat, but is completely swept away from those places which have been exposed to lake or sea erosion. It could not exist in its peculiar position on the tops of spurs, &c., if they had been washed by the sea since it was laid down. Further, if it had been a marine deposit it should have covered the whole landscape irrespective of its form, and it is unlikely that it has been removed by denuding agents from so many places and left comparatively untouched on the spurs and the sides of valleys. I am therefore inclined to think it was a wind deposit during the steppe conditions of a higher land and drier climate, with severe windstorms sweeping from great river-beds greater clouds of dust than are seen now in the Rakaia and Waimakariri, although these are by no means of insignificant proportions at the present time.
The deposit of loess covers up the old shore platforms on the south-west side of Banks Peninsula, therefore the depression during which they were formed was pre-loess, and therefore before the great glacier extension. If this is really so, it serves to emphasize the recency of this extension. The general order of events would therefore be a period of low land, when the marine terraces were formed, then an elevation in glacier times, followed by a depression till now, with probably minor periods of slight elevation. there is a slight elevation going on now, as may be seen from the wave-worn caves at Summer now several feet above high-water mark, and the bands of sand-dunes between Christchurch and the sea. This, no doubt, accounts for the low,
broad terraces to be seen in the lower reaches of the Avon and Heathcote Rivers.
The elevation of the land is always considered a most important point in causing terrace-development, but this is chiefly in those cases where rivers have been near their base-level. Subsequent elevation causes them to form terraces owing to the restoration to them of their power of corrosion. This is the case of the Avon and Heathcote terraces just mentioned.
Now, the Canterbury rivers have a remarkably steep grade, and a depression of the land would hardly be felt in their upper and middle portions. I think it very probable that if the land were lowered till the shore-line corresponded with the main line of railway, the erosive power of the streams near the gorges would be only very slightly altered. Further, if terracing were due to elevation it should be progressive upstream from the coast, whereas the contrary is the case: the terraces are highest in their upper portions.
I do not think that change in the height of the land has materially affected the erosive power of these rivers. Near the sea-coast it has undoubtedly exerted some influence, and the raising of the bed of the Waimakariri near Belfast is most probably due to continued depression of the land.
The Volume and Load.
Other causes must therefore be sought to explain the river terraces. If we consider change in volume, we are forced to conclude that our rivers have shrunk in volume from what they were in the glacier epoch. If our mountains were higher, they would intercept more snow, and the average volume of the rivers would be greater. The largest rivers of Canterbury, such as the Waitaki and Rakaia, drain the highest portions of the Alps; further, the Rangitata, with a comparatively small draingebasin, is nearly as large a river as the Waimakariri with a large drainage-basin, because the small area supplying the Rangitata is an area of high mountains, where the glaciers are larger. Our rivers are therefore smaller than they were, and they would not be likely, therefore, to be able to terrace their beds were this not accompanied by a marked diminution in the load.
It is important to notice here the different grade of the plains—that is, of the old glacier rivers as compared with the grade of the present rivers. They are all, without exception, running on a gentler gradient now than formerly. If we except the hypothesis of elevation along as axis through the outer range of mountains, we are forced to conclude that the last important cause—viz., the load of the river—is the predominating factor in determining whether the rivers could terrace the plains.
or not. The volume in all probability is now less, the grade of the rivers is less, and yet terraces are formed on a tremendous scale.
The existence of enormous supplies of waste in the valley of a river profoundly influences its action. The energy of a stream is limited, and its excess is chiefly spent on transportation and corrasion. It will therefore be evident that terrace-forming must be connected in some way with the load a stream carries. If the load is excessive, there will be no energy left for lowering its bed, and hence for forming permanent terraces. Many of the laws governing streams may be studied by examining the miniature fans and deltas formed at the roadside or in other places after heavy rain. The following order of events is apparently true for a minature fan as for our large rivers:—
During flood-time the stream is fully loaded with waste from the surrounding country, but drops it on the gently sloping ground, thus raising its bed. Terraces are absent. When the height of the flood is past, the supply of waste falls off—only smaller particles are carried; and there is an excess of energy left over for corrasion, and the fan is terraced, on a small scale it is true, but the processes and the sequence of events are just the same as on a large scale. If this is so, the degradation of its bed by a river which is fully loaded in flood-time will occur principally as the flood is falling, and will continue till the river is running clear again and carrying no sediment. I have re-peatedly noticed this order of events on shingle fans, and I have received confirmation of these facts from engineers whose business it is to supervise the fords across the streams on the Christchurch-Hokitika Road. It must be remembered that our rivers when in flood are undoubtedly highly charged with waste, and therefore differ greatly from the condition of ordinary streams when in flood. These may be discoloured by fine particles, and may even move stones along; but the supply of waste on the Canterbury mountains is exceptional in amount, therefore our rivers in flood-time are comparable to the excessively charged streams of a small fan, and the sequence of events is apparently the same, although the conditions are somewhat different.
I think it can be proved that when the volume of a stream diminishes, the transporting power falls off in a slightly greater ratio than the energy. The result of this will be that, when a stream is fully loaded, on a diminution in volume there will be an excess of energy left over for corrasion, and the stream will therefore channel its bed. The explanation of this phenomenon may be due to the fact that with a falling volume the larger
particles are dropped first, and if there is not an approximately equal quantity of smaller material for the river to move in place of the material dropped there will be an excess of energy left over for corrasion. Under ordinary circumstances there is an insuffcient supply, and so the river-channel is lowered.
The supply of waste has such an important bearing on the corrasive power of a river that a consideration of the circumstances which control the supply in the Canterbury mountains will be relevant here. One of their most striking features is the vast supply of débris supplied by their slopes exposed to frost erosion. This effect is so marked that whole mountainsides are covered with angular débris, which is continually moving downwards, but especially so in the case of shingle-slips. These are often from 2,000ft. to 3,000ft. in height, and may be as much as a mile wide. The reasons for this excessive supply of waste are as follows: (1.) The jointed character of the rocks in the drainage-basins of the rivers. (2.) Owing to intense folding of the rocks, they frequently dip at very steep angles, and therefore the weakest beds are exposed to the atmosphere without being protected by more resistent beds. (3.) The age of the folding dates back to Mesozoic times, and therefore weathering agents have been able to exert their influence to a marked extent. (4.) The range, both annual and diurnal, of the temperature is very great. (5.) The absence of close plant-covering over large areas. All these causes promote extensive disintegration, and any explanation of the life history of our rivers must take them into account.
One of the principal factors determining the production of waste is the extent of mountain-slope not protected by a close covering of vegetation. the area of most vigorous denudation is between the snow-line and the upper limit of this covering. The snow protects the underlying rocks to a certain extent; but, nevertheless, even here the denudation is rapid, but especially on those steep faces where snow cannot lie. When the snow is turned to ice the effect is somewhat similar. Erosion will not proceed as rapidly under the ice as on the slopes at a higher and lower level free from ice, but exposed to the action of frost. The effect of elevation of the land will be to make the area above the snow-line greater and to expose a much greater area to the influence of frost. The part affected in the Southern Alps is principally that between the 3,500 ft. and the 7,000ft. contour lines. If the land were raised, the country affected would be approximately that between the same levels, but the area included would be very much greater; although this would be diminished by the accumulation of ice in hollows where it could not melt. Large areas below the snow-line would be
covered with glaciers; but, in spite of this, the area exposed to frost action would be more extensive, and therefore the supply of waste would be in excess. A very large amount of erosion due to glaciers, as estimated by the proportion of sediment in the rivers flowing from their terminal faces, is due primarily to the action of frost on the hillsides above the glaciers.
The supply of waste in this case would be increased during elevation, owing to the previous loosening action of the plants on the rocks rendering them subject to other weathering agencies; again, if this were also attended with a general desiccation of the climate on the mountains fronting the east, the supply of waste would be further increased owing to the disappearance of the protective plant-covering.
From a general survey of the country in the upper basins of our rivers I am of opinion that the period of maximum weathering has passed. The old and mature shingle-slips are far larger than those now existing. Vegetation in many cases has got the better of the moving shingle, and in some cases the old fans are completely covered with forest. Our shingle-slips at the present time are diminishing in extent, and they will continue to do so unless the plant-covering is destroyed either by nature herself or by man.
The excess of waste during a period of elevation accounts for the present form of the Canterbury Plains. They have been formed by the overlapping fans of great glacier streams, as can be conclusively proved by carefully contouring their surface. The contour lines show them to have been formed in exactly the same way as an ordinary shingle fan, except that their grade is more gentle. They were built up to their present height when the rivers were overloaded with sediment, during a time of high land, severe glaciation, and acute frost action. On the land being depressed, the supply of waste would fall off, and the rivers would begin to terrace their old deposits in a manner analogous to that in which a stream terraces its fan during a falling flood. This action was certain to occur unless the volume of the river fell off in a relatively greater proportion. I believe that such would not occur in Canterbury, owing to the excessive amount of waste which would be poured into the rivers falling off in a greater ratio than the decrease in snow or rain.
It will be noted in every case that the grade of the rivers is less than that of the plains; the rivers are therefore able to do their work on a gentler slope than formerly. This can only be due to—(1.) Elevation of the interior of the country since the plains were formed. (2.) Rivers having a greater volume, and therefore power to move their load on a gentler grade: this is extremely unlikely. (3.) A diminution in the supply of waste:
this last appears to me the most satisfactory explanation. No doubt the erosion of its bed which the river is enabled to perform owing to the diminution of the supply of waste would tend to be neutralised by the depression of the land proved on page 32. If the land had been low, and the former supply of waste comparatively small, this depression would have been sufficient to produce aggradation instead of corrasion. But the land is still high, the rivers are still powerful torrents, and the supply of waste fast diminishing. These factors are sufficiently great to nullify the effect of depression in the higher portion of the river course; but the rivers have now reached such a stage in their development that in their lower course aggrading is now going on: hence depression has made its influence apparent. This is what might reasonably have been expected; and, if depression continues, this effect will become more and more marked, so that the terraces will tend to disappear. However, should the slight elevation which has taken place recently continue, aggrading in the lower portion of the river-course will cease and terracing will be resumed.
I have been confirmed in my conclusion that the supply of waste is a controlling factor in the terrace-development of our rivers by observation of the history of shingle fans. In their youthful stage they are built up by an aggrading stream; in their vigorous middle period they are partly channelling their fans and partly building them up on their outskirts; when they reach their mature stage they become channelled and terraced by the stream that runs through them. This terracing closel [ unclear: ] resembles that on the plain course of our rivers. It is more marked near the apex of the fan, and falls off towards the fringe. This may be due to the fact that the river is more confined near the apex of the fan, and therefore more capable of vertical corrasion. But it is also due to the fact that in former times of excessive supply of waste that waste was chiefly deposited just below the gorge. It may perhaps be due to increase in volume of the river as it enlarges its drainage-area. Howeover, increase in volume will not explain the fact that after every freshet a stream apparently terraces its fan on a diminishing volume.
In his accounts of the formation of the Canterbury Plains, Captain Hutton maintained that they had been levelled by the sea and subsequently raised, so that the rivers were able to terrace them. If this were the case, terracing should progress up-stream, should show a maximum development near the sea, and not, as in this case, near the gorges. If, however, the loess is not marine but of æolian origin, as seems very probable, and since it is incapable of resisting marine erosion, there can-
not have been any recent elevation of more than a few feet. The general recent direction of land movement has been down-ward, and this is indicated also by the aggradation going on in the Lower Waimakariri and Rakaia.
The evidence afforded by Otago, where river-terracing is also shown on a gigantic scale, points distinctly to a sinking land. Unless there has been at the same time an increase in the rainfall—and as long as conditions have been the same over the Tasman Sea there seems to be no reason why this should have increased on the mountains—we are at once driven to consider the supply of waste to be a predominating factor in terrace-formation in the valleys of the Canterbury rivers. If we consider those parts of the world where terraces are greatly developed—e.g., British Columbia, the Rocky Mountains region, the Himalayas, and Patagonia—we must be struck by the fact that they have all passed through a severe glaciation, when waste filled the valleys, and now terracing is actively going on. Elevation of the land has had an important effect in some cases, but not in all. It seems that too little consideration has been given to the control exerted by excessive waste-supply.
I have omitted mention in the above of the effect which sagging of the coast-line might have had on the formation of terraces. Owing to the loading of the coast-line with enormous quantities of waste from the land, it is highly likely that differential lowering of the crust has taken place, and is probably going on now; perhaps the general lowering since the glacier maximum may be intensified in the coastal regions by this process. It is highly likely that a large syncline has been forming under the Canterbury Plains and to seaward of them, dating from some time posterior to the Upper Cretaceous period, and that the coalmeasures and overlying limestones and other beds have experienced the results of this movement. Very interesting evidence on this point has been afforded by the cruise of the steam-trawler “Nora Niven.” Mr. Edgar Waite, Curator of the Canterbury Museum, informs me that at certain positions along the coast large pieces of brown coal were brought up in the trawl. They were frequently from 2 ft. to 3 ft. in length, and weighed at times over 1 cwt. They were obtained from the following station: No. 39, twenty-six miles east of Timaru; depth, 28–31 fathoms. No. 42, thirty-one miles north-east of Timaru; depth, 21–24 fathoms. No. 54, twenty-seven miles north-east of Godley Head; depth, 21–27 fathoms. No.57, four miles east-south-east of Waiau River; depth, 26–43 fathoms. Their occurrence at such a uniform depth, their absence else
where, and their large size renders it highly improbable that they were carried to these places either by ocean-currents or by rivers. In fact, pieces of coal of such size would be quickly reduced to fragments in any of the rivers which cut through the coalmeasures. It seems, therefore, highly probable that such masses have come from outcrops of coal in positions which come to the level of the sea-bottom in the localities where they are found.
Similar occurrences of coal outcropping on the sea-bottom are recorded from the North Sea. If this is really so, then the brown-coal measures of Malvern, Mount Somers, and of other places along the foot of mountains probably extend eastward under the plains in the form of a great syncline, and reappear at a depth of about 150 ft. below sea-level about thirty miles to the eastward of the present coast-line on the scarp of the continental shelf. It is therefore likely that sagging of the crust has gone on in Post-Cretaceous times, but with periods of depression and elevation, as proved by the marine terraces on Banks Peninsula. If this has gone on recently, it would no doubt affect the form of the terraces; but I am inclined to think that its effect is not apparent, unless the depression of the land which went on since the glacier maximum is partly due to this cause. The effect of this depression is, without doubt, apparent in the lower courses of the present rivers, as explained previously.
In conclusion, I have to express my sincere thanks to the following gentlemen for their kindly criticism and generous advice and assistance on many points: Dr. L. Cockayne, Dr. F. W. Hilgendorf, Messrs. E. G. Hogg, E. K. Mulgan, R. M.Laing, T. H. Jackson, Edgar R. Waite, and Edward Dobson, C.E.
The following is a list of papers, &c., which have been referred to in the above, or which have some direct bearing on the subject:—
Haast, Sir J. von: “Geology of Canterbury and Westland, with a Special Chapter on the Formation of the Canterbury Plains.”
Haast, Sir J. von: “On the Geology of the Canterbury Plains.” Trans. N. Z. Inst., vol. vi.
Thomson, J. T.: “On the Glacial Action and Terrace-formation of South New Zealand.” Trans. N.Z. Inst., vol. vi. (This paper draws special attention to the resemblance between the mode of forming a river-fan and that of the plains.)
Crawford, J. C.: “On the Old Lake System of New Zealand, with Some Observations on the Formation of the Canterbury Plains.” Trans. N.Z. Inst., vol. viii.
Hardcastle, J.: “Origin of the Loess-deposit of the Timaru Plateau.” Trans. N.Z. Inst., vol. xxii.
Cox, S. H.: “Report of the Geological Survey, Mount Somers and Malvern Hills District, 1883.” (This gives a good description of the geological structure of the rocks in the districts named, and in particular those near the Rakaia Gorge.)
Hutton, Captain F. W.: “On the Cause of the Former Great Extension of the Glaciers in New Zealand.” Trans. N.Z. Inst., vol. viii.
Hutton, Captain F. W.: “Note on the Silt-deposit at Lyttelton.” Trans. N. Z. Inst., vol. xv.
Hutton, Captain F. W.: “On the Lower Gorge of the Waimakariri.” Trans. N.Z. Inst., vol. xvi.
Hutton, Captain F. W.: “The Geological History of New Zealand.” Trans. N.Z. Inst., vol. xxxii.
Hutton, Captain F. W.: “On the Formation of the Canterbury Plains.” Trans. N.Z. Inst., vol. xxxvii (1904).
Hutton, Captain F. W.: “Report on the North-east Portion of the South Island.” Geological Survey Report, 1872–3.
Hutton, Captain F. W.: “The Origin of the Fauna and Flora of New Zealand.” “Annals of Natural History,” vol. xv (1885).
In these articles Captain Hutton puts forward his views as regards the reason for the extension of the glaciers, the evidence for the marine origin of the loess, and for the formation of the Canterbury Plains. As they come from such a distinguished author, they are worthy of the greatest consideration.
Cockayne, Dr. L.: “The Plant Geography of the Waimakariri.” Trans. N.Z. Inst., vol. xxxii.
This paper gives an excellent account of the present climatic conditions of the basin of the Waimakariri, as well of its Œcological botany. Special attention has been paid to the xerophilous plants, and to the reasons for their frequent occurrence in this area.
Hilgendorf, Dr. F. W.: “The Influence of the Terrestrial Rotation on the Canterbury Rivers.” Trans. N.Z. Inst., vol. xxxix (1906).
This paper is a valuable contribution to the literature dealing with the river-terraces. In it the author attempts to prove that the earth's rotation has affected the form of the terraces. While admitting that this is a vera causa, yet the geological difficulties in the way of conclusively demonstrating its effect are so great that I cannot regard the conclusion as satisfactorily established. The labour and care which the author has displayed in collecting his data are worthy of admiration, and this paper will always remain a standard one with reference to the form of the cross section of the river-beds from terrace to terrace, whatever the cause of the form may be.
Dr. Albert Heim, Professor: “Neujahrsblatt von der Naturforschenden Gesellschaft auf das Jahr, 1905, Neuseeland.’ Zürich, 1905.
Looking south-west through the Rakaia Gorge. The terrace in the fore-ground has been eroded largely from solid rock, outcrops of which can be seen on its level surface in three places.
Upper Waimakariri. Partially truncated spur, taken from the top of another on opposite side of river-bed, which is here about three-quarters of a mile wide.
Looking down Rakaia River from the Gorge Bridge, showing river-bed and high terraces.