e Truckee Canyon glacier
was joined at the town of Truckee by a short but powerful
tributary, which, taking its rise in an immense rocky
amphitheater surrounding the head of Donner Lake, flowed
eastward. Donner Lake, which occupies the lower portion of
this amphitheater, was evidently formed by the down-flowing
of the ice from the steep slopes of the upper portion near the
_summit_. The stage road from Truckee to the summit runs
along the base of a _moraine_ close by the margin of
the lake on one side, while on the other side, along
the apparently almost perpendicular rocky face of the
amphitheater, 1000 feet above the surface of the lake, the
Central Pacific Railroad winds its fearful way to the same
place. In the upper portion of this amphitheater large patches
of snow still remain unmelted during the summer.

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