E. Interpretation of Observations.
The foregoing is a statement of the facts as I see them relevant to the question of the extent of glacierisation of the locality, and the following is a summary of the conclusions to be drawn therefrom.
The first point to be considered is the distribution of the large angular and sub-angular blocks of the highest terrace north of the river. The concentration off the southern end of the Surrey Hills near Montalto, rising as it does above the general surface of this terrace, appears to be abnormal if regarded entirely as an alluvial deposit. Its form, notably the bank flanking it on all sides, and specially facing the plains, is against this contention (Plate 31, Fig. 1). It is reminiscent of the low mounds composed of rounded gravels, which margin morainic deposits in other places, such as between The Point and Glenroy in the case of the Rakaia, and its flat top indicates that this has been planed by stream action, but if the blocks which veneer its flanks be regarded as having been derived from a morainic deposit not completely dispersed and buried by streams, then the whole occurrence, and specially the scarp facing the plains, can quite well be accounted for.
Further south-west and more in a line with the glacial streams as they issued from the gorge, any such deposit would feel the full force of their destructive action, and one would expect it to be scattered, or destroyed, or buried in the gravel of the river fan, the cross-section of which rises to the very edge of the highest terrace facing the river, as can be seen from the contour lines on the map. But in spite of this exposure to destruction the scarp which commences near Trig. U persists as a definite feature. This persistence appears inexplicable if the form and especially the grade of the plains is attributed to river deposition, but it is what might be expected if the glacier front and its moraine were in close proximity and were exposed to the wash of water. The large blocks on the tread of the second main terrace near Moorhouse's Road are close to the
extension of the line of this scarp, and may have some relation to it. It should be noted that no block of greywacke or associated rock from the back country was found on the southern end of the Surrey Hills and such blocks should be easily detected if they lay on the surface of the volcanics forming these hills, so it is reasonable to conclude that the ice did not reach as far as their lower slopes.
It would be a simple solution of the problem to regard the veneer of boulders on the face of the top terrace as having been dropped initially by a glacier retreating from a more advanced position and included subsequently in the material forming the top layers of the terrace, the larger blocks sinking gradually in the loosely compacted gravel as happens at the present time in front of many glaciers in New Zealand, and also in the case of concrete blocks washed from the protective works on the banks of gravel-bearing rivers. The scouring of the finer material either on the downstream or the upstream side of the block is largely responsible for its settlement and ultimate disappearance. If this is correct, one must explain why large blocks lie thickly on the treads of the lower terraces, and also why a veneer of coarse material masks a finer-grained alluvial basement. I can only suggest that the underlying gravels, as they form part of the lower strata composing the plains, have had an opportunity to become more compacted, and therefore more capable of supporting larger blocks on a water-worn surface; such a compacted surface will be less liable to scour. The upper layers of the top terrace have not had this chance of becoming consolidated, and the larger blocks have sunk into them when washed by water, only to appear on the edge of the terrace when the river has cut down to a lower base-level and has removed the finer material forming a matrix in which the larger blocks are included. Of course, if the glacial river be considered competent to move such blocks for a considerable distance from the ice-front and to concentrate them into definite accumulations, then it must be conceded that the terrace affords no positive proof of the former presence or proximity of ice, but the break near Montalto facing the plains has still to be explained, and its somewhat irregular line, with embayments facing upstream, cannot be accounted for by subsequent erosion by a river flowing along its front.
When considering this aspect of the matter, it must be remembered that at times glaciers do not form terminal moraines, and the conditions that determine their absence are not thoroughly understood. In this case a paucity of moraine may be due to a glacier issuing from a somewhat confined valley and deploying on to a plain where it would tend to spread, thin out, and develop an indefinite front, and the material it carried through the gorge would be scattered as a thin veneer over a wide area. The paucity of evidence in this case may be accounted for in some such way.
Another problem in this connection is the origin and subsequent history of the blocks that veneer the treads and risers of the lower terraces (Plates 31 and 32). They may have been derived from material included in the top terrace when its surface extended right across the present site of the river, the large blocks contained therein merely sinking to a lower and lower level as the stream eroded its bed deeper and deeper into the pre-existing cover. But in all probability
this does not represent the entire case. As noted earlier, the lower main terrace ends upstream in a moraine, which is at a much lower level than the uppermost terrace on the north side of the river, so the river must have cut down to the level of the second main terrace before the moraine was deposited, the vertical erosion taking place during an interglacial interval. It must be noted, however, that the cutting down of a terrace is often associated with glacier advance in areas outside those where glaciers operate, but this case is within the glacial area and the cutting down has been interglacial. The formation of terraces during a glacier maximum is frequently credited to the lowering of the sea-level and consequent stream base-level when the water has been taken from the ocean to provide the ice-cover of the land, but the terrace formation in the case of the Canterbury rivers in their course across the plains can hardly be attributed to eustatic changes, for the terraces are all lowest near the sea and rise steadily when followed inland. If they were due entirely to change in sea-level this should not be the case unless the change were accompanied by a tilting movement along an axis parallel to the western edge of the plains, and of this there is no evidence at present available, although the removal of the ice-cover and the consequent crustal adjustment has been mentioned as a possible explanation.
It is therefore probable that the river had cut down its channel to the level of the second main terrace during an interglacial interval and that there was a subsequent advance of the ice, certainly as far as the moraine inside the gorge, and perhaps much further. The accumulation of large angular masses on the surface and flanks of this terrace certainly suggests the proximity of ice. It seems hardly possible that they could not have been moved to their present positions from an ice-front near the mouth of the gorge by water alone. It is admitted that they have been subject to strong water action, but this could have taken place as the ice retreated from a more forward position. The size of the masses suggests that the ice passed the site of Mr. McIlwraith's farm, and there is also the accumulation of blocks up to seven feet in diameter just beyond Moorhouse's Road. Could a stream of the size of the Rangitata or perhaps a little bigger have moved and concentrated such blocks when it was entirely unconfined and free to wander over the surface of a plain with an inclination of about 50 feet per mile? It is possible, but I hardly think it probable. I therefore conclude that the glacier extended about six miles from the mouth of the gorge.
I should like to refer very briefly to the evidence as to the maximum extent furnished by the other glaciers that deployed on to the western margin of the Canterbury Plains during the Pleistocene period. This is most complete in the case of the Rakaia, since undoubted moraine, travelled blocks, boulder clay with scratched stones, and rounded hills composed of gravels, occur some six miles out on to the plains from the outlet of the gorge of the river. This glacier was larger than the others since it was fed by a greater length of the snow-clad main divide, and also the outlet of the valley was wider than the others and this allowed freer egress for the ice-stream.
The Ashburton Glacier had its origin in Mount Arrowsmith (9171 feet), a peak lying some 10 miles east of the main divide, and it was reinforced by the Cameron Glacier, also rising in Mount Arrowsmith, as well as by an overflow from the Rakaia coming along the line of Lake Heron and the Clent Hills, and by a distributary from the Rangitata mentioned earlier in this account (page). This enlarged glacier reached the vicinity of the Mount Somers-Springburn Railway, for accumulations of large stones like those in the Rangitata occur near the township of Mount Somers, and rounded hillocks similar to those marking the front of the Rakaia Glacier lie just beyond the line of railway. The analogy of the evidence to that of the Rangitata is perhaps due to the fact that after coming through a somewhat narrow gorge and over the hills in its vicinity it spread out on the lower country and thus lost concentration.
The conditions obtaining in connection with the Waimakariri were still more closely analogous to those of the Rangitata, for the glacier came through a narrow valley after leaving the great intermontane basin whose floor it occupied, and no terminal moraine, washed or unwashed by the glacial stream, is to be found either in the gorge or on the plain near its outlet. When the glacier reached the plains, if, indeed, it really did so, it must have deployed on to a wide area, and thus the proof of its former presence has become as indefinite as that of the Rangitata.
In conclusion, I should like to express my indebtedness for valuable assistance received from Messrs. F. Langbein and T. G. Beck, District Engineers of the Public Works Department, Christchurch, and specially to the latter for permission to use a map prepared in connection with the survey for the Rangitata Diversion Race. I have also to thank Mr. G. Stokell for substantial help in connection with the examination of the area under consideration.