129

This, at first, seems rather astonishing, for at least three miles from its terminal face upwards, the debris is rounded and exhibits very few large blocks.

During a careful examination of the boulders forming these deposits, I was not able to detect any eruptive or volcanic rocks or debris of the tertiary deposits: at the base of the latter, but only the different sandstones, slates, flagstones, pebble beds and conglomerates which form the Southern Alps, whilst the rivers flowing through these zones now bring down a great quantity of volcanic detritus, from which we may conclude, that when these deposits of the glacial period were formed, the volcanic mountains (never more than 5000 feet, and generally 3000 to 3550 feet high), were lying below the level of the sea.

In order better to understand what I have to say about deposits of a similar nature in the Alps, it will be necessary to offer a few remarks on their physical geography. The main chain of the Alps, as far I explored them, runs north-east and south-west, and projects several divergent chains, often very little inferior in size, in a north and south direction. The main glaciers run along the base of the Alps in a line parallel to the main chain, either in a southerly or northerly direction. From the spot, where the rivers leave their glacial cave, they are true shingle rivers, that is to say, they meander through a straight valley often three miles broad without any falls or even great rapids, although their general fall is from 40 to 50 feet in the mile. The valley generally opens until they reach the last boundary chain of the plains, through which the rivers have cut deep lateral gorges with nearly perpendicular rocky walls, through which they rush with great impetuosity. In these gorges we find only traces of the drift period.

By following attentively the rising deposits, I found invariably, that above the gorges they again recurred, and by calculating the distance and the average rate of rise, which is from 35 to 40 feet in the mile, I found they corresponded perfectly, so that, in fact, the deposits in the middle courses of the rivers form a continuation of those of the Canterbury plains, consisting of the same boulders, but larger and more angular.

Several eminent geologists have entertained an opinion, that deposits forming similar plains, could not be brought down by glacial action, because the boulders are for the greater part rounded, but I am induced to arrive at a different conclusion from the fact, that the old glaciers were of a much larger size than the present ones, and that where brought down by the latter, the debris very soon presents a similarly rounded appearance.

For instance, on the great Tasman glacier, which, at its terminal face, has still a breadth of one mile and three quarters, we find that the surface of the glacier resembles the channel of a river bed, and, in fact, during the melting of the snow, and during and after heavy rains, the water rushes in streams over its surface, whilst below the centre of the glacier, at its end, a channel covered with debris shingle is formed, which is usually perfectly dry. Even at a few yards distance from the glacier, the debris carried down to its face when exposed to the action of the streams very soon looses the sharp angles and assumes the shape of the boulders which we find in the drift deposit. It is evident that the same agency which rounds the debris near the glacial caves and along the terminal face of the glaciers themselves, are also at work at the bottom of the sea.

Either currents rolled them, or when shallower water occurred so as to occasion the stranding of icebergs, the boulders were acted upon by the waves, which, from recent researches, have been found to act on substances deposited on the sea bottom at a depth of several hundred feet. Clay deposits (glacial mud, formed principally by attrition) mark those spots when the sea was undisturbed, either by under currents or by the action of the waves, and the fact that amongst them I several times observed angular debris, deeply imbedded, affords an additional proof of the manner of their deposition.

But when we enter the Alps we soon find some very interesting phenomena which further explain the formation of drift deposits. The spurs which branch from the central chain, for many miles upwards, have been acted upon by two different agencies. The first and older of the two agencies was the sea, the effects of which are palpable from the fact that during the drift period the country was sunk at least 5200 feet below the sea level, above which it afterwards rose at several different periods so as to form ten distinct main terraces, which occur between the altitudes of 2500 and 5200 feet, giving to each terrace an average height of 290 feet, but these terraces are sometimes subdivided so as to present no less than 30 distinct smaller ones, as regular in appearance as fortifications.

They are only intact where we see no trace of glacial action after the period of their upheaval, as, for instance, in the Ashburton Plains on both sides of Lake Heron, but when we examine the main valleys at the head of which glaciers are still found, level terraces, when not destroyed by the atmospherics, are only met with at the lower ends of the chain, soon disappearing below the lateral moraine lines, formed during the later glacial period.

Both terrace formations are easily recognisable, not only by the deposits of which they are composed, or by which they are covered, showing their littoral or moraine character, but also by their slopes. Whilst the marine terraces, as before stated, are quite horizontal, the moraine terraces slope down-