58 TOPOGRAPHIC DEVELOPMENT OF KLAMATH MOUNTAINS, [bull. 196. 
upon sandstones which are possibly Mesozoic. Although Miocene 
is well developed a short distance farther up the coast, it was com- 
pletely removed at this point before the deposition of the Pliocene, 
indicating an epoch of elevation and erosion between the Miocene 
and Pliocene. The extent of the erosion during this interval is 
unknown, but, as already pointed out by Le Conte, 1 there are 
indications of submarine valleys extending seaward from the present 
coast to the continental border, 2 and it is possible that they were cut 
at this time. 
GLACIATIOX OF LATEK VALLEYS. 
The later valleys, as already noted, are often canyon-like, especially 
near the valley bottom and the coast. Toward the head waters the 
valleys are generally shallower and more open, and have an aspect of 
greater age than the portion near the coast, and yet their difference 
in age is probably so small as to be scarcely measurable in geologic 
time. 
Tracing the larger streams up into the Siskiyous, the Yallo Ballys, 
or the rugged peaks about the head of Salmon and Trinity rivers, one 
finds the valleys to be glaciated to an unexpectedly large extent. On 
the northeast slopes of both North and South Yallo Bally Mountains, 
near the line between Tehama and Trinity counties, Cal., there were 
formerly glaciers a number of miles in extent which have left well- 
defined records in striated and polished rocks and ground moraines, 
with small lakes and meadows above terminal embankments. These 
ancient glaciers are represented to-day by large snowbanks, resulting 
from the protection which the mountains afford against the driving 
southwest winds of the winter storms. 
Important ancient glaciers among the mountains about the head of 
Salmon and Trinity rivers have been noted by Mr. Oscar H. Hershey. 3 
The largest of these — Swift Creek Glacier — is described as follows: 
At its maximum extension this glacier had a length of not less than 15 miles, a 
width of \ to 1 mile, and a depth of 1,000 to 1,500 feet. It was the largest single 
mass of ice, so far as I know, of the Sierra Costa Mountains. It headed among 
the peaks in the highest portion of this range, at an altitude now about 6,500 feet, 
trended in a northeasterly direction, forming the broad flat of the Mumford 
Meadows (altitude 5,500 feet), then ran southeasterly, descending rapidly to a 
level now little more than 3,500 feet above the sea, where at 10 miles from its 
head it suddenly issued from the high mountains and. turning to the northeast, 
deployed upon and across a broad basin valley of Miocene age and later, 4 and 
terminated very close to the site of the Redding and Trinity Center road, at an 
elevation now no greater than 2,500 feet above the sea. 
1 Bull. Geol. Soc. America, Vol. II, p. 325. 
- Lawson (Bull. Dept. G-eol. Univ. California, Vol. I, pp. 57-59) regards these as due to faulting, 
and not to erosion, 
a Jour. Geol., Vol. VIII, 1900, pp. 42-57. 
4 This valley is the northeastern extension of the Weaverville Basin (see page 44). 
