The only other agency to which we could attribute this transportation is that of shore ice and icebergs, which probably did once exist on Lake Tahoe; but the limitation of the pebbles to the western, and especially the northwestern shores, is in exact accordance with the laws of glacial transportation, but contrary to those of floating ice transportation—­for lake ice is carried only by winds, and would, therefore, deposit equally on all shores.

    Again:  I think I find additional evidence of a Lake Tahoe
    “mer de glace” in the contrasted character of the northern and
    southern shores of this Lake.

All the little glacial lakes described above are deep at the upper end and shallow at the lower end.  Further, all of them have a sand beach and a sand flat at the upper end, and great bowlders thickly scattered in the shallow water, and along the shore at the lower end.  These facts are easily explained, if we remember that while the glacial scooping was principally at the upper end, the glacial droppings were principally at the lower end.  And further:  that while the glacial deposit was principally at the lower end, the river deposit, since the glacial epoch, has been wholly at the upper end.

Now the great Lake, also, has a similar structure.  It also has a beautiful sand and gravel beach all along its upper shore, and a sand flat extending above it; while at its lower, or northern end, thickly strewed in the shallow water, and along the shore line, and some distance above the shore line, are found in great abundance bowlders of enormous size.  May we not conclude that similar effects have been produced by similar causes—­that these huge bowlders were dropped by the great glacier at its lower end?  Similar bowlders are also found along the northern portion of the eastern shore, because the principal flow of the ice-current was from the southwest, and in the fulness of glacial times the principal exit was over the northeastern lip of the basin.

b.  Origin of Lake Tahoe.  That Lake Tahoe was once wholly occupied by ice, I think, is certain; but that it was scooped out by the Lake Valley glacier is perhaps more doubtful.  All other Sierra lakes which I have seen certainly owe their origin to glacial agency.  Neither do I think we should be staggered by the size or enormous depth of this Lake.  Yet, from its position, it may be a plication-hollow, or a trough produced by the formation of two parallel mountain ridges, and afterward modified by glacial agency, instead of a pure glacial-scooped rock-basin.  In other words, Lake Valley, with its two summit ridges, may be regarded as a phenomenon belonging to the order of mountain-formation and not to the order of mountain sculpture.  I believe an examination of the rocks of the two summit ridges would probably settle this.  In the absence of more light than I now have, I will not hazard an opinion.[3]

[Footnote 3:  This question practically has been settled by Mr.
 Lindgren, and his conclusions are given in an earlier chapter.]