(b) South Bank.
The highest terrace (1a) on the south bank of the river starts from just below the junction with Boundary Creek, and follows along the base of the foot-hills of the Mount Peel Range in a south-easterly direction across Rawle's and Chapman's Creeks to just above Lynn Creek and the vicinity of the Mount Peel homestead. It is over a mile wide between Rawle's and Chapman's Creeks, but north and south of them it has experienced the destructive action of streams coming in from the south as well as the inswing of the river at various levels, so that near Boundary Creek it shows only as small detached fragments. This terrace is higher than the highest terrace north of the river, and in the vicinity of Rawles Creek its surface is some 425 feet above river level (aneroid reading), and its face is 225 feet high. There are small remnants of a shelf forty feet higher still, but these have been modified by wash from the neighbouring hills so that a determination of their true height is illusory. The structure of the main terrace is well displayed on the banks of the tributaries from the south which have incised it deeply and have steep banks, and, in my opinion, the material belongs to an older very late Tertiary or early Pleistocene gravel series, for the material is smaller than that of the undoubted river terraces, is less rounded, more consolidated, contains much clay in places, and is also at times well stratified. No glacial material appears to be associated with it.
Mention should be made of two isolated shelves below Lynn Creek whose upper surface is slightly higher than that of the main terrace upstream. They are composed of the same gravels, etc., as the main terrace, as can be seen on the high banks of the Lynn and of a sub-parallel creek to the south-east, and the apparent increased height, which amounts to thirty feet at most, may be attributable to the raising of the surface of the terrace by the fans of two tributaries when they entered the main river at a higher level, a modification described by Cotton (1940, p. 28) in connection with terraces in the Esk Valley of North Canterbury. This modification naturally increases the height of the terrace most in close proximity to the point where the tributary leaves the steep wall of the valley to debouch on to its floor, and when the terrace is cut back subsequently by the lateral erosion of the river it leaves a remnant out of accord with the general level of the terrace.
Below the terrace just referred to, and near its Boundary Creek end, there are short terrace remnants, and then some 225 feet below it, that is, 200 feet above river-level, lies the best-developed terrace (2) south of the river. This is broad and accordant with the second
main terrace north of the river. It extends downstream from Boundary Creek past Chapmans Creek to the vicinity of Mount Peel homestead, where it has been cut off by the inswinging of the river at a subsequent date when it destroyed the upper terrace as well and left the small shelves near Lynn Creek. The two parts now separated by the river form the most striking physiographic feature of the locality. Opposite the ravines where Rawle's and Chapman's Creeks and other tributaries issue from the base of the upper terrace the surface has been modified by former high-level fans of these tributaries according to Cotton's suggestion (loc. cit.). Measurements made with an abney show that the surface may be raised as much as fifty feet, an amount which diminishes progressively in the direction of the margin of the terrace. This gives some idea of the extent to which the level of a terrace may be raised consequent on lateral fan formation. Subsequently the terrace has been trenched by these tributaries as the main river has lowered its base-level, and their high banks show that the terrace has been cut to some extent, if not altogether, in the same material as composes the highest terrace.
It resembles its northern counterpart in the occurrence and distribution of large blocks. While they show only occasionally on the tread of the terrace considerable accumulations lie on the edge and down the riser slope facing the river, and they continue as far downstream as the terrace is preserved, a noteworthy occurrence being about two miles from the Mount Peel homestead. Included in this concentration is a large block of granite-conglomerate seven feet in diameter. This lies over two miles upstream from the site of a similar collection near the end of Moorhouse's Road on the opposite bank of the river. Wherever clear-cut sections can be seen the large blocks rest on rounded gravels and are not embedded in them (Plate 30, Fig. 2).
Lower level terraces are not developed as fully on the south side as on the north side of the river. They occur upstream from the Mount Peel homestead, above and below the infall of Rawle's Creek, while near the intake of the race is an interesting suite of smaller terraces, occasionally mere shelves, five in number in some places and seven in others (Plate 30, Fig. 2), the discrepancy being due to the removal of portions of terraces already formed by the inswinging of the river at lower levels. All these terraces near the intake are veneered with large blocks, while at the intake itself the bed of the river is floored with large blocks, and I am assured by the Resident Engineer that they did not change in position to the slightest extent during the record flood in February of this year (1940). If the blocks now lying in the somewhat confined bed of the present river have not been moved by a record flood, it seems unlikely that those at a higher level on the terraces, where the streams would have been less confined and not so deep, though perhaps larger in volume, could have been carried an appreciable distance to their present positions by water alone.
On the south bank of the river near the intake is an interesting occurrence not obviously connected with glaciation, viz., a discordance in level of about four feet which affects the lowest two terraces of the suite (Plate 32, Fig. 2). Had this affected only one terrace
Map of the country in the vicinity of the outlet of the Gorge of the Rangitata River. The part north of the river is based on the survey for the Irrigation Diversion Race, with slight alterations. That south of the river has been sketched in, but the position of the two main terraces is reasonably correct; these are numbered 1 for the top main terrace and 2 for the lower main terrace; other terraces are largely diagrammatic.
Upper part of the Rangilata [ unclear: ] intermont view looking north-west from Ross's Cutting near Potts River, the bed of which shows in the foreground.
Edge of the break in grade near Trig. U. just south of the Surrey Ilills, view looking north, sleep slope 45 feet high facing the plains.
Morainic blocks on edge of intermediate terrace. 15 feet above No 2, north side of the river, view looking downstream.
The outlet of the Rangitata Gorge looking north-west; Boundary Creek in the middle, the river coming in on its right; the site of the intake is in the middle at the bottom of the picture; the face of the lower main terrace (No. 2) on the north bank lying on the right, and the whole suite of terraces on the south bank on the lett. The trace of the fault-scarp can be seen leading up from the river-bank on the lett.
it might have been attributed to some accident in river erosion, but the fact that it affects two terraees, and on the same line, discounts this explanation, so it must be attributed to faulting. Some difficulties remain, however, for the dislocation affects higher terraces of the suite to a less and less extent, and the uppermost are not affected at all, nor does the break show on the north side of the river where it might be expected, unless a slight waviness in the surface of the second main terrace can be attributed to this cause. The greywacke basement is not exposed on the line of the break so the effect on the solid rock cannot be seen. The dislocation is clearly posterior to the formation of the lowest terrace, so it must date from comparatively recent, perhaps even from historical times, and it furnishes another indication that tectonic movements have continued in the Southern Alps down to the immediate past, and in all probability have not ceased yet. Its occurrence here extends the area where recent faulting has been recognised. Due weight was given to this proof of crustal instability in the locality when the mode of formation and the accumulation of angular masses in the gorge, i.e., the moraine, was under consideration.