ACADEMY OF SCIENCES. 171 
of the south fork of the American River terminates in Johnson’s Pass, a gap 
in the western summit of the Sierras, about 7,500 feet above the level of the 
sea. The break in the mountains extends for about a mile and a quarter 
from north to south, and is nearly level. The upper part of the American 
Valley, for three or four miles, rises by a gentle slope up the pass, and is from 
half a mile to a mile wide, with a flat meadow bottom of mountain meal, 
bounded on each side by moraine blocks, lodged against the sides of the 
mountains. The head of the pass terminates by a pretty steep escarpment 
which forms a part of the western boundary of Lake Tahoe Valley, lying 
about 1,000 feet below. The south side of the American Valley, near the 
pass, is formed by a mountain about 9,000 feet high, the face of which, oppo- 
site the head of the pass, suddenly changes its direction, turning to the south 
to form the west wall of the southern termination of Lake Valley. To the 
north, the pass is separated from Echo Lake Valley by a vast bank of moraine 
matter, which formed at one time a la‘eral moraine of Echo Lake glacier, but 
which has been subsequently increased and gradually sloped off towards the 
valley by the bed of the glacier being forced up over it during the middle of 
the glacial epoch. As this Echo Lake glacier has evidently been an important 
element in causing the glacial action at the upper part of the American Val- 
ley, a short description of its old bed will be useful. The Echo Lake Valley 
is about four miles long, running in a direction southeast, northwest, and 
terminating towards the northern or upper end in a perfect amphitheater, sur- 
rounded by high peaks. The chord of this amphitheater or cirque measures 
probably two miles and a half, affording ample area for the formation of a 
vast glacier. The valley abuts to the southeast against the western wall of 
Lake valley at the north termination of the depression which forms Johnson’s 
Pass. The bottom of the valley is now occupied by two lakes, one of which 
is a mile and three-quarters long, and a quarter of a mile broad, with a depth 
of water of 150 feet. The other, or upper lake, is smaller and not more than 
thirty-five feet deep. They are separated by a belt of rock a few yards broad, 
in which the granite presents a more schistose character. The rocks on the 
border of the lake show evident marks of glacial action to a height of 400 
feet above its level, and moraine matter has been deposited fully 200 feet 
higher, so that the Echo Lake glacier must have been between seven and eight 
hundred feet thick. During the earlier part of the glacial epoch it was pre- 
cipitated over an almost perpendicular cliff, a thousand feet high, into Lake 
Valley, and whilst pursuing this course it piled up a vast moraine on its south- 
west border, the upper end of which terminates at a height of about 300 feet 
above the level of the lake. This now forms the divide between Echo Lake 
and the American River Valley. The true nature of this embankment is well 
shown where it terminates above Lake Valley, and also at its upper part 
where it joins a spur of the mountain, but the greater portion of it has been 
so completely covered in by detritus, under the form of mountain meal, and 
the few rocks that appear on the surface are so completely rounded and 
polished that but for the appearances at the upper and lower ends, its true 
character as a lateral moraine would not be suspected. 
As before stated, the lower end of Echo Lake Valley terminates at the edge 
of the escarpment forming a portion of the sides of Lake Valley, and it 
