My examination of these two glaciers, however, was very cursory. I hasten on, therefore, to others which I traced more carefully.
Lake Tahoe lies countersunk on the very top of the Sierra. This great range is here divided into two summit ridges, between which lies a trough 50 miles long, 20 miles wide, and 3000 to 3500 feet deep. This trough is Lake Valley. Its lower half is filled with the waters of Lake Tahoe. The area of this Lake is about 250 square miles, its depth 1640 feet, and its altitude 6200 feet. It is certain that during the fullness of glacial times this trough was a great "mer de glace," receiving tributaries from all directions except the north. But as the Glacial Period waned--as the great "mer de glace" dwindled and melted away, and the lake basin became occupied by water instead, the tributaries still remained as separate glaciers flowing into the Lake. The tracks of these lingering small glaciers are far more easily traced and their records more easily read, than those of the greater but more ancient glacier of which they were once but the tributaries.
Of the two summit ridges mentioned above the western is the higher. It bears the most snow _now_, and in glacial times gave origin to the grandest glaciers. Again: the peaks on both these summits rise higher and higher as we go toward the upper or southern end of the Lake. Hence the largest glaciers ran into the Lake at its _southwestern end_. And, since the mountain slopes here are toward the northeast and therefore the shadiest and coolest, here also the glaciers have had the greatest vitality and lived the longest, and have, therefore, left the plainest records. Doubtless, careful examination would discover the pathways of glaciers running into the Lake from the eastern summit also; but I failed to detect any very clear traces of such, either on the eastern or on the northern portion of the western side of the Lake; while between the southwestern end and Sugar Pine Point, a distance of only eight or ten miles, I saw distinctly the pathways of five or six. North of Sugar Pine Point there are also several. _They are all marked by moraine ridges running down from the summits and projecting as points into the Lake_. The pathways of three of these glaciers I studied somewhat carefully, and after a few preliminary remarks, will describe in some detail.