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.