CHAPTER I

LAKE BASINS AND THEIR RELATION TO GLA• CIERS-THE PARALLEL ROADS OF GLENROYESKERS, ETC.

IN the preceding chapters we have endeavoured to collect the principal facts relating to ice and its work. The one gives a sketch of the scenery, and describes the deposits in a land which was formerly overspread with ice. For this Switzerland was selected rather than Scandinavia, because its situation permits us to attribute all its deposits, dating from the Glacial Epoch, to the action of land-ice; for it is generally admitted that, since this epoch began, the whole country has been above the sea. The other chapter attempts to depict regions where a Glacial Epoch now prevails, in which also the work of ice, both on land and by sea, can be studied. Greenland, for this purpose, is exceptionally instructive, because it must reproduce very closely the conditions prevalent in the north-western part of the European continent during the Glacial Epoch, and thus must throw light on certain difficult problems which the latter presents. These we pass on to consider. The first-the vexed

question of the part played by large masses of ice in the excavation of lake basins-has a less direct connection with Greenland, and will be discussed in the present chapter, together with other collateral subjects. The second practically amounts to this— How far can the work and deposits of land-ice be distinguished from those which are more immediately due to floating ice? This question can be most conveniently discussed in connection with the history of the British Isles during the Glacial Epoch, and is thus reserved for the next chapter. Each question at the present time is a battle-field in geology, and though it will be convenient to consider the two apart, the one cannot be wholly separated from the other. In either case we shall do our best to state the facts which have been ascertained, to give the rival interpretations, and to indicate the points in each which appear to be strong or weak, when they are regarded in the light of results established by the study of cases where either there can be no controversy or the differences of opinion, comparatively speaking, are but slight.

In the year 1862 the late Sir A. Ramsay1 sought to prove that the lakes in and about the chain of the Alps had been excavated by the ancient glaciers. The following is a brief summary of his paper, which, whether the conclusions of the distinguished author be ultimately accepted or not, did great service

1 Quart. Jour. Geol. Soc. xviii. (1862), p. 185.

in clearing away misconceptions by which the inquiry previously had been much obscured :—

(1.) None of these lakes lie in simple synclinal troughs, that is to say, in synclines the axis of which has a general correspondence with the longer diameter of the lake basin. As the author says: "The lake hollows in the Alps are encircled by rocks, the strikes, dips, and contortions of which often exhibit denudation on an immense scale; and in no case is it possible to affirm, here we have a synclinal hollow of which the original uppermost beds remain.” Neither do they lie in any area of special subsidence, such as might be produced by the dissolving and removal of underlying masses of rock. Though this occasionally might be a cause, the number of adjacent dimples in the earth's crust, which this hypothesis not seldom would demand, and the fact that the basins often lie among rocks which are practically insoluble, is fatal to it as one of general application.

(2.) The Alpine lakes cannot be hollows eroded by the rivers which still pass through them.

“Run

ning water may scoop out a sloping valley or gorge, but (excepting little swallow-holes) it cannot form and deepen profound hollows, so as to leave a rocky barrier all round."

(3.) The Alpine lakes do not lie in lines of gaping fracture. This idea, which found much favour with some of the older geologists, is disposed of conclusively by showing that even the deepest and narrowest of

these lakes, when drawn on a true scale, bears no resemblance whatever to a fissure. Since the date of Sir A. Ramsay's paper many sections of the beds of lakes have been published, founded on more accurate and much more numerous observations than existed in 1862, and these have placed the accuracy of his reasoning in this respect beyond question.

These hypotheses, as the most probable, having been considered and dismissed, the author shows that the Alpine lakes lie in the paths of the glaciers or ice-sheets which once radiated from the mountains; that they are "broad or deep according to the size of the glaciers that flowed through the valleys in which they lie, this general result being modified according to the nature of the rock and the form of the ground over which the glacier passed." In some cases the presence of a lake may be determined by a sudden increase in the thickness of the ice owing to a confluence of glaciers in a restricted area, such as a rather deep and steep-sided valley, or by some other cause the exact nature of which cannot now be recognised. Sir A. Ramsay also pointed out that in all regions of the Northern Hemisphere which unquestionably have been subjected to great glaciation, such as many parts of Scandinavia, Finland, the Outer Hebrides, the Highlands of Scotland, and districts of North America, "the whole country is covered with a prodigious number of lakes." Accordingly, all other explanations fail

ing, he concludes that one agency alone remains, "that of ice, which from the vast size of the glaciers ... must have exercised a powerful erosive agency. It required a solid body, grinding steadily and powerfully in direct and heavy contact with and across the rocks, to scoop out deep hollows" in situations. such as have been already mentioned.

1

These arguments were subsequently re-stated by Dr. A. R. Wallace, who urges an additional one, which is thus expressed :-"If we look at the valley lakes of our own country and of Switzerland, the first thing that strikes us is their great length and their situation, usually at the lower end of the valley, where it emerges from the higher mountains into comparatively low country." Basins of glacial erosion do not invariably occur in mountain valleys, because they require a combination of favourable circumstances, but three criteria may be formulated by which such basins are distinguished from ordinary valleys(1.) They never present those peculiarities of contour which are not infrequent in mountain valleys, and never exhibit either submerged ravines or those jutting promontories which are so common a feature in hilly districts." (2.) "Alpine lakebottoms, whether large or small, frequently consist of two or more basins, a feature which could not occur in lakes due to submergence, unless there were two or more points of flexure for each depression,

1 Fortnightly Review, November and December 1893.