The Rangitata Glacier.

This glacier was also of considerable size, having been about 48 miles long. It reached during its greatest extension several miles into the Canterbury plains, crossing the front ranges—before the present lower gorge was cut—by a saddle to the south of it. After its retreat to higher regions a lake was formed behind the front range. This lake in course of time was partly filled up by alluvium, partly drained by the formation of the lower gorge. Morainic accumulations, numerous glacier shelves, terraces and roches moutonnées in the middle Rangitata, bear ample testimony to the wide-spread glaciation of the country during that remarkable period. The Rangitata glacier was anastomosing with the united Rakaia-Ashburton glacier by the large opening in which now Lakes Tripp and Acland are situated, receiving afterwards again a branch of the latter by Trinity Valley. After the partial retreat, it continued to send a branch into the former mentioned broad opening, where several frontal moraines round Lake page 391Tripp are still well preserved. The arrangement of the beds, owing their deposition to glacier action can be well studied on the banks of the lower Potts, a tributary of the Rangitata, crossing diagonally the Lake Tripp valley, and of which section No. 11, plate 7, gives the details. Several erratic blocks lie on the surface, below which bed No. 1 appears. It is about 60 feet thick, and consists of morainic accumulations, containing a large number of enormous angular blocks of rock imbedded in yellowish silt. It is separated by a sharply defined line from No. 2 grey moraine, also full of large angular blocks in which the interstices are filled up by coarse greyish detritus, thickness about 15 feet; No. 3, yellow moraine, also replete with huge blocks, of which, however, a number have rounded angles, thickness about 90 feet; No. 4, very hard fissile, yellow and white fine glacier silt, having a horizontal position, some of the layers being as thin as a sheet of paper. This bed has doubtless been deposited in a glacier lake. It changes towards its upper portion gradually into sand and fine gravel; towards the bottom it is however sharply defined, thickness about 40 feet. It reposes on No. 5 grey moraine, consisting of a great number of mostly large and perfectly angular blocks, thickness about 100 feet. Below this last deposit a bed of a thickness of about 80 feet is visible, consisting of till, sand and silt, alternating repeatedly with each other. The character of the beds for the lowest 100 feet could not be ascertained, as they are covered by a talus of debris from the deposits above. It is thus clear that at different times various conditions prevailed here. The upper deposits are either lateral moraines of the Potts glacier branch once joining the Rangitata glacier, or they belong to the latter when it divided here in two branches, of which one travelled towards the Ashburton opening. No. 3 was probably a ground moraine. The peculiar conditions of this assembly of beds go far to prove that glaciers when advancing again after their retreat, do not always clear out their former channels of morainic deposits accumulated therein, and that even under favourable circumstances the finest glacier silt, when protected by moraines of comparatively inconsiderable thickness, is so thoroughly protected that no change in its stratification can take place.