Art. XLV.—Some Notes on the Marlborough Coastal Moraines and Waiau Glacial Valley.
[Read before the Otago Institute 4th October, 1910.].
The Marlborough coastal moraines extend from the shores of Cook Strait near Cape Campbell southward to Shades Creek, a distance of twenty-six miles. I have elsewhere* spoken of the three greatest of these glacial deposits as the Cape Campbell Moraine, the Kekerangu Moraine, and the Shades Creek Moraine, but a recent examination of the maritime belt in which they occur has led me to the belief that these moraines, although now more or less detached, at one time constituted a continuous deposit, having
[Footnote] * James Park: “The Geology of New Zealand,” 1910, p. 201.
been formed by an ice-sheet that descended from the Kaikoura Mountains to what is now the shore-line of the north-east portion of the South Island.
There is indisputable evidence that the confluent glaciers which formed this ice-sheet extended eastward beyond the present strand, but just how far cannot be determined until more data are available.
At the mouth of Kekerangu River the moraine-material rises into bold headlands over 400 ft. high, and, moreover, forms rocky ledges and reefs that can be traced seawards for some half a mile, until they disappear in the deep water.
Three miles north of Kekerangu River, at Shaw's Falls, there is a considerable development of morainic material; and at many points between that place and Lake Grassmere, and between Lake Grassmere and the Wairau Valley at Blenheim, much glacial débris of a similar character is to be seen.
In the Kekerangu Valley, about two miles from the sea, a strip of glacial moraine composed of material identical with that found in the coastal moraines is entangled in a powerful fault that follows the foot of the range. This faulted moraine is beautifully exposed in the steep banks of Heaver's Creek (see Plate XX).
At the mouth of Shades Creek there are two bands of morainic material—namely, a narrow band entangled in a fault-plane close to the point where the main coastal road crosses the stream, and a broader band parallel with the first and about 30 chains to the westward. This second greater band varies from 10 to 40 chains wide, and extends from the south side of Shades Creek northward two miles or more towards Kekerangu, crowning and forming the coastal ridge which culminates at Deadman's Hill, 1,070 ft. high.
The Shades Moraine is splendidly exposed to view in the sea face of Deadman's Hill, along the foot of which the Blenheim coach-road passes southwards to Kaikoura. Enormous blocks of grey Amuri limestone are
seen projecting from the steep slopes fronting the sea; and half a mile from the sea the course of Shades Creek is blocked up and rendered impassable with piles of fossiliferous boulders that have fallen from the adjacent morainic ridges (see Plate XXI).
The presence of such a vast accumulation of ice-formed and ice-borne detritus on the present shore-line offers a temptation to speculate as to the former existence of an ice-sheet that may have stretched far beyond the present strand. Of a great extension beyond the present shore-line there is, however, not much evidence. An examination of the detritus shows that it is mainly fluvio-glacial and glacial, such as might very easily be formed at or near the edge of a piedmont ice-sheet.
The morainic reefs off the mouth of the Kekerangu River lie at their furthest limit half a mile from the shore. They may possibly extend as submerged reefs another half-mile or more to seaward; but, notwithstanding this possibility, I am inclined to think that the present strand marks approximately the eastern extension of the Pleistocene glaciers during the period of maximum refrigeration.
The Kekerangu and Shades Moraines, although so far north, rank among the finest examples of glacial deposits in the South Island, and among the most accessible. They are, unlike the Cape Campbell Moraine, close to the main coach-road connecting Flaxbourne and Kaikoura, and can be reached from either Blenheim or Kaikoura in a day.
A prominent feature of these moraines is the presence in them of large angular blocks of different rocks plucked from the bed-rock in the neighbourhood or transported from a distant watershed.
In the face of the cliff forming Homai, the south headland at Kekerangu (Plate XXII), there are many large angular blocks, among which two conspicuous masses of Amuri limestone* immediately catch the eye when standing on the beach. The most northerly mass measures 32 ft. by 30 ft. by 20 ft.; but it should be mentioned that these dimensions may be much greater, as the distance the block extends into the cliff cannot be seen. The southerly block measures 38 ft. by 20 ft. by 11 ft., the last dimension being the distance the mass is seen to project beyond the cliff-face.
Some 25 yards south of the last mentioned block of Amuri limestone, and at about the same height, there is a large mass of soft blue foraminiferous clay from the Awatere beds near Flaxbourne or Seddon, which measures in the two dimensions that are exposed 30 ft. by 22 ft.
Besides these, this moraine also contains large slablike masses of soft Tertiary sandstone, with numerous fossil shells, and blue shaly clay with leaf-impressions, the largest slab seen by me measuring 10 ft. long, 6 ft. wide, and 2 ft. thick.
The included blocks of rock in the Shades Moraine are more numerous and larger than at Kekerangu. In the face of Deadman's Hill, as seen from the beach, three masses of Amuri limestone are particularly conspicuous. The largest measures in the two dimensions that are exposed 72 ft. by 24 ft.; the second, 51 ft. by 28 ft.; and the smallest and most southerly, 46 ft. by 32 ft. by 30 ft., the last dimension not being fully exposed.
Mount Vernon Drifts.
These form the Vernon Hills, lying from two to five miles south of Blenheim, and extending from the sea along the west side of the Wairau Valley
[Footnote] * These are shown in fig. 93, p. 202, “The Geology of New Zealand,” by James Park, 1910. Photo by A. McKay, F.G.S.
for over twenty miles. They consist of a great pile of fluviatile drifts over 1,200 ft. thick. Towards their base the drifts are intercalated with beds of clay and soft sandstone alternating with beds of gravel. At their base, and resting on the basement rock, which is mainly greywacke and jointed argillite of probably Lower Secondary age, there is a rock-rubble deposit mainly composed of angular blocks of greywacke, few of which exceed 2 ft. in diameter. The material is loosely piled together, and in many respects resembles the terminal moraine of the Hooker Glacier or the lateral moraines of the Tasman. It varies from 10 ft. to 160 ft. thick, and is specially well exposed on the Awatere side of Maxwell's Pass.
The Vernon drifts, with their basal glacial débris, form the divide between the Lower Wairau and Lower Awatere, rising in places to a height of over 2,000 ft. above the sea. In places they rest on the older Pliocene clays of the Awatere series. Like the Shades and Kekerangu deposits, they are in places tilted at high angles, in others traversed by faults. That they are Pleistocene seems almost certain.
Waiau Glacial Valley.
The southern portion of Marlborough is traversed by the Inland and Seaward Kaikoura chains, which are separated by the gorgelike Clarence Valley. Ten miles north of the Kaikoura Peninsula the seaward chain pursues a south-west course, which carries it further and further inland, so that when it crosses into the prolongation of the Province of Nelson that protrudes itself between North Canterbury and South Marlborough it is no longer a seaward but an inland range of mountains, still forming, as it does further north, the southern wall of the Clarence Valley.
Five miles south of the Kaikoura Peninsula there begins another coastal range that extends southward to Cheviot and North Canterbury. Between this coastal range and the Seaward Kaikouras, now an inland chain, there lies a wide, well-defined valley extending from the Upper Waiau and Hanmer Plains to Kaikoura. When standing at the Hanmer River it is at once seen that this valley was the course followed by the ancient Waiau Glacier as it flowed towards the sea, and also of the Waiau River before the cutting of the present gorge.
The evidences of prolonged and intense glaciation are everywhere present from the Hanmer Plains to the sea at Kaikoura Peninsula, in the shape of smooth, flowing, ice-shorn contours, truncated spurs, spurless ridges, isolated ice-grooved and whalebacked hills remaining in the floor of the glacial valley, roches moutounées, and moraine débris.
The Waiau glacial valley is easily identified, as the inland coach-road from Kaikoura to the Upper Waiau follows along its floor for the greater part of the way. It is not now drained by a trunk river, but is crossed transversely by a number of large streams that descend from the southern slopes of the Seaward Kaikouras, and reach the sea by deep, narrow gorges cut through the coastal range. Beginning at the Kaikoura end, we first have the Kahautara River and its branches, then the Conway River, and lastly the Mason River, with its tributary the Lottery. These rivers have deeply dissected the ancient glacial floor, which, however, can be clearly traced at Greenhills, Highfield, and Mason.
Near the north end of the chain of roches moutonnées running along the floor of the glacial valley on the south side of the Conway, and locally know as the “Whaleback,” there are piles of morainic débris mainly composed of angular blocks of Amuri limestone and Saurian greensand mingled with
greywacke drift. The angular blocks appear to have been plucked from the south end of the Whaleback, where they occur in situ, and carried to their present place, a distance of six or eight miles. Exposures of this glacial débris, but not the best, are seen on the coach-road soon after crossing the Conway going southward.
In the overdeepened portions of the glacial valley there occur considerable deposits of stratified sands and clays, in places containing seams of impure lignite. These deposits were probably formed in the period of fluviatile activity that everywhere throughout the South Island appears to have followed the retreat of the Pleistocene glaciers.
The Kaikoura Peninsula lies at the seaward end of the Waiau glacial valley, the course of which is about north-east to south-west.