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.