Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/296

Rh 282 many medial lines of moraine, so many indeed as some- times to be entirely covered with debris to the complete concealment of the ice. At such parts the glacier resembles a bare ﬁeld or earthy plain rather than a solid mass of clear ice of which only the surface is dirty with rubbish. At the point where the glacier ends, the pile of loose materials is tumbled upon the valley in what is called the terminal moraine. In such comparatively small and narrow iee-sheets as the present glaciers of Switzerland, the rock-bottom on wluch the ice moves is usually, as far as it can be examined, swept clean by the trickle or rush of water over it from the melt- ing ice. But when the ice does not ﬂow in a mere big drain (which, after all, the largest Alpine valley really is), but overspreads a wide area of uneven ground, there cannot fail to be a great accumuhition of rubbish here and there underneath it. When the broad central plain of Switzer-- land between the Alps and the J11ra was ﬁlled with ice, the latter certainly pushed a vast deal of mud, sand, and stones over the floor of the valley. This material is known to Swiss geologists as the moraine profonde or Grundmorc'ine.1 When from any cause a glacier diminishes in size, it may drop its blocks upon the sides of its valley, and leave them there sometimes in the most threatening positions. Such stranded stones are known as perched blocks. They abound in the Swiss valleys, extending even across the great plain of Switzerland, and appearing in numbers high upon the ﬂanks of the J nra. Since the latter moun- tains consist chieﬂy of limestone, and the blocks are of various crystalline rocks belonging to the higher parts of the Alps, the proof of transport is irrefragable. The agent of removal is now recognized to have been an enormous ex- tension of the glacier system of the Alps, whereby the whole country was buried under snow and ice. Similar evidence abounds in the valleys among the mountainous parts of Britain, as well as in other parts of Europe and America, no longer the abode of glaciers. b. Erosion-.—The manner in which glacier ice erodes its channel differs in many respects from that in use by any other geological agent, and forms therefore one of the dis- tinguishing features of ice-action. This erosion is effected not by the mere contact and pressure of the ice upon the rocks, though undoubtedly fragments of rock must now and then be detached from this cause. It is by means of the ﬁne sand, stones, and blocks of rock, which fall between the ice and the rocks on which it moves, that the grinding work of the glacier is done. These materials, held by the ice as it creeps along, are pressed against the rocky sides and bottom of the valley so ﬁrmly and persistently as to descend into each little hollow and mount over each ridge, yet all the while moving along steadily in one domi- nant direction with the general movement of the glaciers. As a. result, the most compact resisting rocks are ground down, smoothed, polished, and striated. The striae vary from such ﬁne lines as may be made by the smallest grains of quartz up to deep ruts and grooves. They sometimes cross each other, one set partially effacing an older one, and thus pointing to shiftings in the movement of the ice. On the retirement of the glacier, hummocky bosses of rock having smooth undulating forms like dolphins’ backs are conspicuous. These have received the name of roches moutomzées. The stones by which this scratching and polishing are effected suffer in exactly the same way. They are ground down and striated, and since they must move in the line of least resistance, or “end on,” their 1 The present writer examined in 1869 a characteristic section of it near Solothurn, full of scratched stones, and lying on the striated pavement of rock to be immediately described as further characteristic of ice-action. GEOLOGY [u1. 1)'.'.M[CAL. strize run in a general sense lengthwise. It will be seen, when we come to notice the traces of former glacier.-, how important is the evidence given by these striated stones. As rocks present great diversities of structure and hard- ness, and consequently vary much in the resistance they off L r to denudation, they are necessarily worn down unerpially. The softer, more easily eroded portions are scooped out by the grinding action of the ice, and basin-shaped or various irregular cavities are dug out below the level of the general surface. Similar effects may be produced by an augmented excavating power of a glacier. as where the ice is strangled in some narrow part of a valley, or where, from change in declivity, it is allowed to accumulate in greater mass as it moves more slowly onward. Such hollows, on the retirement of the ice, become receptacles for water, and form pools, tarns. or lakes, unless indeed they chance to have been already ﬁlled up with glacial rubbish. It is now some years since Professor A. C‘-. Ramsay drew attention to this peculiar power of land—ice, and aﬂirmed that the abundance of ex- cavated rock-basins in northern Europe and America was due to the fact that these regions had been extensively eroded by sheets of land-iee. This glaciation was due not to independent glaciers but to the pressure and grinding power of vast masses of continental ice. In short the more northern parts of Europe and North America must have been in a conditio11 like that of North Greenland at the present day. It is therefore among the ice-ﬁelds of Green- land rather than among the valley glaciers of isolated mountain-groups that we ought to look for analogies to the operations which produced the widespread general glacia- tion of the period of the rock-basins. A single valley- glaeier retires towards its parent snow—ﬁeld as the climate ameliorates, leaving its roc/zcs moulomzées, moraine—mounds, and rock-basins, yet at times discharging its water—drain-age in such a way perhaps as to sweep down the moraine—mounds, ﬁll np the basins, bury the ice-worn lunnmocks of rock, and strew the valley with gravel, earth, sand, and big blocks of rock. Hence the actual floor of the glacier is apt to be very much obscured. But in the case of a vast sheet of land—ice covering continuously a wide region, there can be but little superﬁcial debris. When such a mass of ice retires it must leave behind it an iee-worn surface of country more or less strewn with the subsoil which accumulated under the ice and was pushed along by it. This infra-glacial detritus forms the G1'1uz(l2no2'[i7ze (moraine prqfomlv) or bottom moraine. Ve know as yet very little regarding its formation in Greenland. Most of our knowledge regarding it is derived from a study of the till or boulder-clay of Britain, which is believed to represent the bottom moraine of an ancient ice-sheet. In countries where true boulder- elay occurs, numerous rock-basins are commonly to be met with among the uncovered portions of the rocks. The abundant ﬁne sediment which gives the character- istic milky turbidity to all streams that escape from the melting ends of glaciers is an index of the amount of erosion unceasingly effected by the ice. From the end of the Aar glacier, for example, though by no means one of the largest in Switzerland, it has been estimated that there escape every day in the month of August 2 million cubic metres (440 million gallons) of water, containing 284,374 kilogrammes (280 tons) of sand. B. OCEANIC W vrnns. The area, depth, temperature, density, and composition of the sea have been already treated of in part ii. Viewed as a dynamical agent in geology, the ocean may be studied under two aspects—( 1) its movements, and its geological work.