188 J. LeConte— Ancient Glaciers of the Sierra Nevada. 
constant process of alternate rough hewing and planing. If the 
rock be full of fissures, and the glacier deep and heavy, the 
rough hewing so predominates that the plane has only time to 
touch the corners a little before the rock is again broken and 
new angles formed. This is the case high up on the cafton 
walls, at: the head of vate dake and Emerald Bay, bat also 
in the caton beds wherever the slate is approach : the 
other hand, the sorts is very hard and solid, and the laces be 
not very deep and heavy, the planing will predominate over 
the rough hewing, and a smooth, gentle billowy surface is the 
result. This is the case in the hard granite forming the beds 
of all the cafions high up, but especially high up the cafion of 
Fallen Leaf Lake, where the cafion spreads out, and extensive 
but comparatively thin snow sheets have been at work. In 
some cases on the cliffs, subsequent disintegration of a glacier- 
olished surface may have given the appearance of angular 
surfaces with beveled corners; but, in other cases, in the bed of 
cafion, and on elevated level places, where large Lege 
blocks could not be removed by water nor by gravity, I ob- 
served the same appearances, under conditions anc forbid 
this explanation. Mr. Muir, also, in his Studies in the Sierra, 
gives many examples of Gulsuked rock-breaking by ancient 
glaciers. 
Angular blocks are mostly, therefore, the ruins of crumbling 
cliffs, borne on the surface of the glacier and deposited at its 
foot. Many rounded boulders also have a similar origin, having 
found their way to the bed of the glacier through crevasses, or 
along the sides of the glacier. But most of the rounded boulders 
in the terminal deposit of great glaciers are fragments torn a by 
The proportion of angular to rounded b 
ers—of upper or air-formed to nether or glacier-formed oa. 
ments, mg ge on the depth and extent of the ice-current. In 
the case of the oe ice-sheet (ice-flood) there are, of 
course, no upper formed or angular blocks at all—there is 
nothing borne on heel surface. The moraine, therefore, consists 
wholly of nether-formed and nether-borne severely triturated 
materials (moraine profonde). The conga are, of course, all 
rounded. ‘This is one extreme. In the case of the thin moving 
ice-fields, the glacierets which still linger fimo the highest 
and shadiest hollows of the Sierra, on the other hand, the mo- 
raines are composed wholly of angular blocks. This is the 
character of the terminal moraine of Mt. Lyell glacier, de- 
scribed in my enter —_ These glacierets are too thin 
and feeble and d to break off fragments—they can only 
bear away what falls on them. This is the other extreme. 
But in the case of ordinary glaciers—ice streams—the boulders 
of the terminal deposit are mixed ; the angular or upper-formed 
