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—­there is nothing borne on the surface.  The moraine, therefore, consists wholly of nether-formed and nether-borne severely triturated materials (moraine profunde).  The bowlders are, of course, all rounded.  This is one extreme.  In the case of the thin moving ice-fields, the glacierets which still linger among the highest peaks and shadiest hollows of the Sierra, on the other hand, the moraines are composed wholly of angular blocks.  This is the character of the terminal moraine of Mount Lyell glacier.  These glacierets are too thin and feeble and torpid 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 bowlders of the terminal deposit are mixed; the angular or upper-formed predominating in the small existing glaciers of temperate climates, but the rounded or nether-formed greatly predominating in the grand old glaciers of which we have been speaking.  In the terminal deposits of these, especially in the materials pushed into the Lake, it is somewhat difficult to find a bowlder which has not been subjected to severe attrition.

CHAPTER IX

THE LESSER LAKES OF THE TAHOE REGION AND HOW THEY WERE FORMED

This is not to be a description of the scores of Glacial Lakes found in the Tahoe region, but an answer to the questions so often asked about practically all of these lakes, as to their origin and continuance.

Rich as our Sierras are in treasures none are more precious than these.  They give one pleasing surprises, often when least expected.  For while the tree-clusters, the mountain-peaks, and the glowing snow-banks throw themselves into our view by their elevated positions, the retiring lakes, secluded, modest, hide their beauty from us until we happen to climb up to, or above, them.