Thus, a glacier is constantly breaking off
blocks and making angular surfaces, and then grinding off the
angles both of the fragments and the bed, and thus forming
rounded bowlders and _moutonnees_ surfaces. Its erosion
is a 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 _canyon walls_, at the head of Cascade
Lake and Emerald Bay, but also in the _canyon beds wherever
the slate is approached_. If, on the other hand, the rock
is very hard and solid, and the glacier 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 canyons
high up, but especially high up the canyon of Fallen Leaf Lake
(Glen Alpine Basin), where the canyon spreads out and extensive
but comparatively thin snow sheets have been at work. In some
cases _on the cliffs_, subsequent disintegration of a
glacier-polished surface may have given the appearance of
angular surfaces with beveled corners; but, in other cases,
in the _bed of the canyon_, and on elevated level places,
where large loosened blocks could not be removed by water nor
by gravity, I observed the same appearances, under conditions
which forbid this explanation. Mr. Muir, also, in his
_Studies in the Sierra_, gives many examples of undoubted
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_ bowlders 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 bowlders_ in the terminal
deposit of _great glaciers_ are fragments _torn off by
the glacier itself_. The proportion of rounded bowlders--of
upper or air-formed--to nether or glacier-formed fragments,
depends on the depth and extent of the ice-current. In the
case of the universal ice-sheet (ice-flood) there are, of
course, no upper formed or angular blocks at all--t