Art. XVII.—Notes from the Canterbury College Mountain Biological Station.
No 2.—The Physiography of the Cass District.*
[Read before the Philosophical Institute of Canterbury, 1st December, 1915.]
Dr. Charles Chilton, who is in charge of the Cass Biological Station, has asked me to furnish a short description of the most important physiographical features of the locality, with a view to its use by students and others who visit the station. I therefore trust that these notes may be of some service, and that they may perhaps form a basis for reference in the case of future biological work done in the locality.
The station is situated at an elevation of 1,850 ft., in the vicinity of the Cass River. This river rises in the Craigieburn Mountains, and follows a tolerably straight course in a northerly direction, at first through a narrow mountain-valley, afterwards through a gently inclined plain—the flood-plain of the river—till it discharges through a short gorge into the Waimakariri River, just below the spot where that river receives from the north the waters of the Hawdon (1,746 ft.). The Cass Valley is bounded on the west by a forbidding rocky ridge running parallel to it, whose most important summits are Mount Misery (5,768 ft.) and Mount Horrible. About a mile from the termination of this ridge it is cut down, and forms a low saddle over which passes the coach-road into the main Waimakariri Valley; this is now known as the Cass Saddle, though in the early days it was called Goldney's Saddle (1,929 ft.). On the east side of the Cass the country is open, but a well-defined line of low hills runs south towards Mount St Bernard (5,509 ft), and divides the area into two distinct valleys. In the westerly one lie Lakes Grasmere and Pearson, and in the eastern one lies Lake Sarah. Lake Pearson is the largest of the three; it is about two miles and a half in length, and nearly half a mile wide at its widest part; but it is almost cut into two in the middle, partly by an old moraine, and partly by huge shingle fans, which approach each other from opposite sides of the lake. It is at an elevation of 2,085 ft. above the sea, and has never been known to freeze over. It discharges by Winding Creek towards the Broken River. Grasmere is a much smaller lake, about a mile in length, and nestling close under the ridge which divides the two valleys. It discharges by the Grasmere Stream, which flows through swampy ground close past the station, into the Cass River. Lake Sarah is a small shallow lake, 21 ft. in depth, lying on the opposite side of the ridge from Lake Grasmere, and discharging into the Grasmere Stream. Immediately to the east of the station,
[Footnote] * The places mentioned in this article are indicated in the accompanying locality map.
rising directly from the lake, is the tussock-clad peak called the Sugarloaf (4,459 ft.), along whose northern base flows the Waimakariri. In this part of its course this river has an average east-and-west direction, its bed being a broad flat floor covered with shingle from side to side, over which the river wanders in anastomosing or braided streams. It is in places over a mile in width, and only narrows in this locality where the Hawdon forces the main stream over to the south bank against the rocky bluff which terminates the ridge to the west of the Cass River. On the northern side of the
Waimakariri the mountains rise to approximately 6,000 ft., clad with dark-coloured southern-beech forest on their lower slopes, which passes up into subalpine scrub and mountain herb-field and fell-field, and finally into bare rock and loose angular debris with which the summits and adjacent slopes are usually crowned (see Plate XV, Fig. 2). These mountains and the Craigieburn Range to the south appear to hem in the country near the Cass, and give it the appearance of being placed in a basin. The origin of this basin is no doubt connected in some way with deformation of the earth's crust, such as faulting and warping; but the details of the landscape are chiefly
due to the modification of such a basin by stream and glacier action. The location of the station affords ample opportunity for the investigation on a minute scale of many of the earth-features produced under such circumstances, and it is to be hoped that these will be undertaken as a corollary to the biological investigation for which the station was primarily established.
The rocks of the neighbourhood consist entirely of folded greywackes, slaty shales, and related sedimentaries of Trias-Jura age, which may be looked on as formed principally from the debris of an ancient granite land lying either to the east or to the west of the present land which constitutes the islands of New Zealand. In places the rocks contain much silica, and become cherty or jasperoid in character, as can well be seen on the hill to the north-west of Lake Sarah, near the Cass Railway-station. The quartzose nature of the rock is so marked at this spot that at one time it was prospected for gold by West Coast miners, and their shafts are still visible on the western end of the hill; no payable gold was encountered. The general strike of the beds as disclosed in the railway and road cuttings is N.E.-S W., but considerable variations in the direction occur within short distances. The dip of the beds is usually at high angles, the result of folding by powerful earth-pressures operating in a S.E.-N.W. direction, these movements having been attended by faulting, as can be seen from the smoothed and slickensided surfaces exposed at times in the cuttings. A most important feature, which has resulted largely from the same cause as the folding and faulting, is the system of joints which penetrate the rocks and render them especially susceptible to disintegrating agencies, such as frost, under whose influence they break into rectangular fragments and furnish the enormous supply of waste which mantles the mountains in the vicinity, and which gradually moves downward to lower levels, supplies the material for fans, and at times impedes and clogs the normal course of streams.
The rocks are not known to contain fossils, although remains of molluscs have been reported from the road-cuttings on the Waimakariri front; but they have been assigned to the Trias-Jura age, from their continuity with, and lithological similarity to, rocks occurring at Mount Potts, on the Rangitata, at Malvern Hills, and at the Clent Hills, which contain plant fossils of undoubted Lower Jurassic age.
The date at which they were folded was probably Late Jurassic or Early Cretaceous, since rocks of more recent age than this are not affected to the same extent, although they, too, have experienced a moderate degree of deformation, showing that folding movements had not entirely ceased at that date. When they were folded a mountain range was probably formed which was base-levelled subsequently by stream or sea action, and on the surface thus produced Tertiary limestones, sandstones, &c., were laid down, such as can now be seen at Castle Hill, forming in all probability a discontinuous sheet over the area. The region was then raised with its veneer of later sediments, with some amount of differential movement and faulting, which resulted in the formation of such intermontane basins as at Castle Hill and the mid-Waimakariri. The tract was then dissected by stream action, which had advanced to an early mature stage when it was subjected to a somewhat severe glaciation. The agency of ice and the subsequent results of the formation of waste by the action of frost on the jointed sedimentaries have been responsible, either directly or indirectly, for its more distinctive landscape features.
A striking character of the area is the arrangement of parallel valleys which lie in between the Craigieburn Range and the forest-covered mountains to the north of the Waimakariri. These are, starting from the north—(1) The main Waimakariri Valley (2) the valley in which lies Lake Sarah, and which continues down Sloven's Creek; and (3) the valley in which lie Lakes Grasmere and Pearson, with an extension down Winding Creek towards Broken River. It is probable that these valleys are of structural origin, and are formed by faults which run in a N W -S E. direction. It is certain that the lower portions of Sloven's Creek and Winding Creek have their directions determined in this way; but it is quite uncertain how far these movements extended to the north, and they may have died out before the heads of the valleys were reached, and therefore in their upper portions the formation of the valleys may be attributable almost entirely to water-action. There does not appear to be any evidence that faulting determined the initial location of the Waimakariri Valley, although it may have done so.
We may take it, then, that before the commencement of the glaciation the drainage directions were as follows: First, there was the main Waimakariri Valley, bounded on the south by the interrupted range of hills of which the Sugarloaf and the ridge stretching behind the present Craigieburn Station are parts. Secondly, there was the valley in which lie the biological station itself, Lake Sarah, the St. Bernard Saddle (where it is partially blocked by moraine), and the upper part of Sloven's Creek. This valley is bounded on the west by the somewhat discontinuous ridges extending from the vicinity of the Cass Railway-station in a southerly direction towards Mount St. Bernard, and continued after a break towards Broken River through the No Man's Land. Thirdly, there is the valley farther west, which contains Lakes Grasmere and Pearson; the valley in which Winding Creek lies may be regarded as an extension of this. In my opinion, then, before the country was glaciated these represented the main drainage-directions. In all probability the ridge extending through Mounts Misery and Horrible, between the heads of the latter two valleys and the Waimakariri Valley towards the Bealey, was in a much more complete condition, and there was no gap through which the present Cass River flows towards the main river, and the Cass River did not exist as such, but probably only as a small stream which rose somewhere near its present site and ran past the Grasmere Station directly into the Grasmere-Pearson Valley. The existing valley of the Cass is, therefore, of more recent formation, and represents the captured headwaters of the streams which discharged into the two westerly of the valleys. The reason for this capture will be given later. The remarkable straight alignment of the sides of the Upper Cass Valley to the west of the Grasmere Station suggests a fault origin for it, but it is more likely due to the erosive action of the ice straightening the walls by the removal of projecting spurs, the marked contrast between a stream-valley with its overlapping spurs and the glaciated portion without them being excellently illustrated by a comparison between the lower levels just above the river and the upper debris-covered slopes. Whatever the reason for the straight alignment of the upper slopes, the actual channel of the river has been determined by water erosion alone. (See Plate XV, Fig 1)
The changes in the direction of drainage in this locality can be attributed almost entirely to the disturbing effects of glaciation, and it may be useful to summarize at this stage the evidence for the former presence of glaciers.
The following features are indicative of the former presence of ice:—
(1.) Morainic Accumulations.—These are well seen on the top of Cass Saddle, in the vicinity of the road; on the slopes of the hill facing the saddle and leading round to the Waimakariri; on St. Bernard Saddle; and at the east end of Lake Sarah, where they are specially well exposed in the railway and road cuttings. There are decided morainic heaps between Lakes Grasmere and Pearson and at the lower end of the latter lake, in this case forming dams behind which the water is ponded, and in the former causing a diversion of drainage. Similar accumulations occur on all the down country in the angle between the Broken and Waimakariri Rivers, especially opposite the mouth of the Esk River. Near St. Bernard Saddle these accumulations have the nature of a boulder-clay, for fine material is mixed with hard tenacious clay, and the angular fragments are frequently striated, giving the deposit the character of a ground moraine.
(2.) Scratched Surfaces.—These are well seen on the hard rocks on the faces fronting the Waimakariri; on the top of Cass Saddle, where the surfaces have recently been swept clean of the protective covering of loamy clay; on the northern slopes of the Sugarloaf, facing the Cass River, immediately above the railway. Disrupted striated pavements can be seen in many places where the rocks have been exposed for considerable time to weathering agencies.
(3.) Roches Moutonnées.—Almost all the rock ridges in the vicinity of the station exhibit this character. They are elongated in the direction of the movement of the ice-streams, and reduced to elevations with flowing outlines; some, too, have had their cross-sections narrowed by the lateral corrasion of the streams of ice moving along them and attacking their sides. As a result of this they frequently present the appearance of a sugarloaf when viewed end-on and of a long ridge when looked at sideways. Smaller elevations related in character to roches moutonnées occur on Cass Saddle, where they form a characteristic dimpled surface. An excellent example of a roche moutonnée is the mound (called Romulus on the map) lying close to the Grasmere homestead.
(4.) The cross-sections of the valleys enumerated above are also evidence of the presence of glaciers. Their features are to be seen most perfectly in the main valley. We have the even alignment of their sides, rising at a steady angle from the broad flat floor to about the grass-line, and then sloping back at gentler grade; the absence of overlapping spurs; the truncation or semi-truncation of spur-ends, such as those on the downstream side of the Bealey and Hawdon Rivers and Cass Saddle itself; hanging valleys, such as that in which Andrews Stream lies: all these features, as well as others, indicate clearly that the land suffered a severe glaciation.
We must regard the area, therefore, as covered with a great sheet of ice, formed from snow in the great collecting-basins at the head of the Waimakariri and its tributaries, and moving down the valley to the east, spreading over the undulating country in the triangle between the Waimakariri and Broken Rivers, while through it projected as “nunataks” the summits of the Sugarloaf and Mount St. Bernard.
The volume of this flow may be judged from the size of the cross-section of the main Waimakariri Valley above its junction with the Hawdon and the Cass. Half-way between this point and the Bealey Township the floor is over two miles in width, and this breadth is continued
with slight diminution up-stream almost to the mouth of the Crow River, about four miles above the incoming of the Bealey River. When one also regards the flare of the valley-walls the actual volume of the basin becomes more impressive. It is no doubt true that this great basin has been enlarged to its present size through the abrasive and excavating power of the ice; but during the ice-flood it would act as a great collecting ground, and materially contribute to the maintenance of the supply of ice, the depth of which, judging from the evidence of ice-action on the valley-walls, must have reached nearly 2,500 ft., even if it did not exceed it, and have covered all the slopes almost to the grass-line.
As this great body moved east it received notable additions from the Hawdon Valley and from Andrews Valley, and at the same time the cross-section of the main valley was much reduced. Hence, driven by the weight of the ice above it and forced south by the tributaries coming in from the north, it crowded over the Mount Misery - Cass Ridge and entered the territory belonging to the valleys lying to the west of the main Waimakariri, lowering the divide continuously by erosion. (See map.) The tendency of such overriding flows of ice to cut distinct channels is well illustrated in the notches in the ridge just mentioned, the upper ones formed at the height of the ice-flood, and the lower one at the Cass Saddle being formed throughout the whole period during which the ice was enabled to surmount that portion of the ridge. Hence it is that the effects of ice are so noticeable in its vicinity, for the erosive action would be very great at that spot. The semi-detached knob which terminates this ridge, and round which the railway goes from the Cass Valley to that of the main Waimakariri, is the remnant of the ridge which has escaped destruction when the overflow cut in behind it. It belongs to one of the “beehive” forms common in glaciated regions where spurs have been semi-truncated The direction of the movement of ice is shown by that of the striae, and also by the slope of the ridges between Mount Horrible and the Cass River. In the basin which lies immediately to the west of the river these trailing spurs all slope down from the north, even against the flow of the Cass River, and discharge their surplus water south This peculiarity can be attributed chiefly to the action of ice scouring out channels and leaving ridges pointing in that direction.
The disturbance in the regular and even flow of the ice-stream where it overrode the end of the Cass spur would be very great indeed, and this would be intensified by the stream from the Hawdon coming in at right angles from the north The effect would closely resemble the eddies or whirlpools seen under similar circumstances in a river. It seems that a great ice eddy occurred between the Cass Saddle and the northern end of the Sugarloaf, and this scoured out a great roundish pothole. A similar landscape form is seen at the junction of the Lake Stream with the Rakaia, formed under exactly similar conditions, but its shape is more circular. In this case, too, a narrow rocky ridge had been left at the outlet of the tributary stream which it has subsequently cut through. The marked similarity of the two cases suggests more than a mere coincidence in shape It is probable, then, that a rockbound hollow marked the site of the junction of the Cass with the Waimakariri, and when the ice retreated this formed a lake divided from the main stream by a rock bar. (See Plate XV, Fig 1)
The flooding of the country with ice in the direction of the Broken River is responsible for all the glacial features indicated previously;
almost all the down country in the angle between this river and the Waimakariri was covered, and at the height of the glaciation the ice reached through the main gorge of the latter river as far as its junction with the Kowhai. Erosion modified the shape of the valleys, and deposits of angular material, such as the moraines at the lower end of Lake Pearson and between that lake and Grasmere, as well as the great mass of angular debris which forms St. Bernard Saddle, marked stages in the recession of the ice-flood.
It must not be assumed that all of this was due to ice which invaded this territory from the Waimakariri. The basin itself must have acted in some measure as a collecting ground, especially the Upper Cass Valley and the hollows of the Craigieburn Mountains; but the area lies too far to the east of the main divide to allow of any great accumulations of snow, most of which would have been intercepted by the higher range to the west. The Craigieburn Mountains do show, however, the shell-shaped hollows at high levels, some of which certainly held small glaciers of the “corrie” type. On this range they are usually filled with debris at the present time, which is frequently piled in rough ridges across the lower end of the hollow—either a morainic accumulation from the glacier period or due to the rolling of loose stones down the frozen snow slopes from the exposed ridges above. In the wetter regions, where plants have a better opportunity for establishing themselves, they form ideal spots for the formation of colonies of alpine plants. The immediate vicinity of the Cass Station is not altogether favourable for examining them, except as to the accumulations of debris, but a short journey to the mountains near the Hawdon enables them to be seen in typical development.
When the ice retreated and left the vicinity of the Cass considerable changes took place in the drainage. These resulted chiefly from two causes: First, the deposit of morainic material caused interference with the normal directions of streams, an interference which might have been slowly overcome had not other causes accentuated it. Transitory lakes, such as Lakes Sarah, Grasmere, and Pearson, probably occupied inequalities in the floors of the valleys, and these would have discharged along the original lines of flow—approximately, at any rate. Now, Lake Pearson does this, but it is slowly disappearing owing to infilling from the sides, and also to its cutting down the dam which blocks it towards Broken River, large areas of drained lake bottom being there visible. Lake Grasmere, however, being more within the sphere of influence of the other cause, has reversed its direction, and has not proved competent to lower the barrier at its southern end. The second important factor has been the overdeepening of the main bed of the Waimakariri in the vicinity of the Cass by the excavating action of the powerful ice-stream which moved down the main valley. The ice here would be much thicker than that in the valley near the middle Cass, and, as power of excavation depends directly on the thickness of the ice, the floor of the main stream would be lowered much below the level of that of the Cass. This overdeepened portion was in all probability occupied by a lake after the ice retreated up-stream towards the Bealey, until the rock bar in the neighbourhood of the mouth of the Esk and below it was removed by erosion. When this occurred the effective base-level of all the streams near the Cass would be lowered also, and it is easy to understand, if the ice had not already formed a gap in the rock bar at the mouth of the Cass, why it was cut through, and, if the gap had been formed by the ice or
by subsequent stream erosion, why the headwaters of the streams in the valleys running south from the Cass were rapidly diverted to the main river, a change which would have been accelerated by the blockage of the lower parts of these valleys by morainic bars and by shingle fans, as will be indicated presently. Thus it is that the Cass has captured the stream issuing from the Craigieburn Mountains near the Grasmere Station, and also the waters of Lake Grasmere and Lake Sarah, and numerous other small streams as well. The resemblance to the conditions of the Lake Stream in the Rakaia Valley is very close indeed in this respect. The presence of the morainic dam south of Lake Heron, and the overdeepening of the main Rakaia to the north, has resulted not only in the capture of small streams in the vicinity of the outlet of the Lake Stream, but also the waters of Lake Heron itself and its feeders, and, most important of all, the Cameron River, all of which belong to the Ashburton basin rather than to that of the Rakaia.* The parallelism between the features of both localities is indeed most remarkable.
The district furnishes excellent examples of shingle fans in all stages of development; in fact, I do not know of any locality in New Zealand where a more representative series can be obtained, the country in the immediate vicinity of Lake Pearson being especially prolific in the varying forms of this landscape feature. A common mode of origin, excellently illustrated by the district, is from the gutters or runnels which sometimes seam the mountain-side for many hundred feet. These are no doubt formed at times by a single heavy shower of rain, and their channels are marked by raised banks and levées, and by a cone of detritus at their lower end on more or less level ground—the deposition of the load of debris being due in both cases to the check in the velocity of the stream, in the first instance by friction with the side and the adjoining vegetation, and in the latter by the lowering of the grade. The deposit at the lower end of the gutter grows as material is supplied, and the runnel enlarges its basin by continued erosion of the sides. As the stream increases in volume the angle of elevation of the fan diminishes and its fringe broadens, and, if space is allowed, its ultimate size will depend on the volume of water and the supply of waste. Fans whose diameter exceeds a mile are by no means uncommon in the Cass region, but they attain greater development in such unconfined areas as the Canterbury Plains. The stream usually occupies the highest radius of the fan, contained by levées on either side, but sooner or later it breaks from this unstable position and commences building on another radius. Construction along successive radii goes on steadily, and the deposit of one overlaps that of another, so that the surface of the fan is raised progressively. The establishment and maintenance of the plant covering has a material influence on its growth, and there is evidently a close connection between the condition of this covering and the stage of evolution of the fan, the geological and the ecological factors acting and reacting on each other. As a rule, when a fan has reached a moribund condition the detrital matter is completely covered with a closed plant formation. The biological station is placed on a fan which is practically dead, and there are good examples of fans in a similar condition on the north side of the Waimakariri, where they are covered partly with forest
[Footnote] *“The Mount Arrowsmith District: a Study in Physiography and Plant Ecology” Trans. N.Z. Inst, vol 43, 1911, p. 341.
and partly with tussock. As a geologist, I should like to know which plant formation was first established on these fans—forest or tussock grassland. The Cass River bed is an active fan on a gentle grade, accommodating itself to the volume of the stream and the supply of waste, both being relatively large. In the valley towards Lake Pearson there are many active and decadent fans, especially on the lower slopes of the Craigieburn Range and Mount St. Bernard. They have spread over the floor of the valley from various points in the sides, dividing it into sections, interfering with the drainage, and thus forming, or helping to form, lakes, such as Grasmere and Sarah, and again filling them up by the lateral extension of the margins of the fan. They are perhaps as important factors in stream-diversion as glaciers, and are the most distinctive agents in the production of landscape features since the ice retreated. I can only suggest, in conclusion, that the position of the biological station affords an admirable opportunity for a detailed study of their true mathematical form and other characters, and a careful investigation into these would furnish results which would be quite worth the time and trouble spent in obtaining them.
The following publications may be consulted as regards special aspects of the physiography of the locality:—
1865. Dobson, Edward. “The Possibility of constructing a Road through the Otira, Gorge.” Report furnished to the Canterbury Provincial Council.
1879. Haast, Julius von. “Geology of the Provinces of Canterbury and Westland.” Christchurch.
1907. Speight, R. “Notes on some of the New Zealand Glaciers in the District of Canterbury.” Report of Aust. Ass. Adv. Sci., 1907, p. 285.
1909. Gudex, M. C. “Some Striated Stones from the St. Bernard Saddle, Upper Waimakariri Valley.” Trans. N.Z. Inst., vol. 41, p. 33.
1915. Speight, R. “The Intermontane Basins of Canterbury.” Trans. N.Z. Inst., vol. 47, p. 336.
1916. Speight, R. “The Orientation of the River-Valleys of Canterbury.” Trans. N Z. Inst., vol. 48, p. 137.