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Collection: Books and Periodicals
The Tertiary Gravels of the Sierra Nevada of California by Waldemar Lindgren (1911) (301 pages)
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Page: of 301
28 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA.
old detrital deposits, until their beds now lie about 100 to 250 feet below their old flood plains.
The alluvial deposits of the present rivers are not extensive near the mouths of the canyons,
but a short distance below begin to widen and finally merge into the large alluvial areas of the
Great Valley. ‘
It is assumed that the deposition of the Quaternary bench gravels was contemporaneous
with the main epoch of erosion which excavated the present canyons and that the glacial epoch
which has just closed in the high Sierra corresponded with the deposition of the present alluvium.
During the erosion of the canyons of the range to depths reaching 4,000 feet and the accompanying removal of enormous masses of the covering Tertiary volcanic formations an amount
of material has been transported into the valley which far exceeds that moved during the
whole period of the Tertiary auriferous gravels. There is no evidence of this tremendous
mass of detritus in the present valley, for the bench gravels described above can account for
only a small part of it. There are no débris fans in the valley corresponding to those resting
in front of the desert ranges in the Great Basin, for instance. It is difficult to avoid the conclusion that the Great Valley has subsided to a corresponding degree since the beginning of
this epoch of erosion. At first glance it looks as if the load had been removed from the range
to the valley which has sunk under its weight.
It has been assumed that the close of the volcanic period marks the close of the Pliocene
and that the erosion of the canyons falls into the early Quaternary, while the glacial epoch
would correspond to the late Quaternary. The paleontologic evidence tends to show, however,
that the volcanic epoch lay within the Miocene, and this would permit the great erosion of
the canyons, the Sierran period of Le Conte, to occupy the time of the Pliocene and the Quaternary. If the close of the volcanic epoch should be removed to the beginning of the Pliocene,
it would give a more adequate length of time for the accomplishment of the gigantic work of
erosion which is manifested in the deep trenches now scoring the flanks of the range.
SUMMARY OF GEOLOGIC EVENTS ALONG THE VALLEY BORDER.
The geologic history of the valley border from late Cretaceous time to the present day
may be summed up briefly as follows:
. Pre-Chico (late Cretaceous) erosion.
. Chico transgression. Shore line moves eastward to present elevations of 300 to 2,000 feet.
Post-Chico erosion. Shore line moves west of present valley border.
Tejon (Eocene) transgression. Shore line moves eastward at least to present elevations of 800 feet.
Post-Tejon erosion. Shore line moves west of present valley border.
. Tone (Miocene) transgression. Shore line moves eastward to present elevations of 1,000 or 1,200 feet.
. Post-Ione erosion.
. Deposition of interrhyolitic shore gravels.
. Postrhyolitic erosion.
10. Deposition ‘of interandesitic shore gravels.
11. Deposition of volcanic ash beds of Oroville.
12. Important period of erosion (Pliocene and Quaternary)—Sierran period of Le Conte. Excavation of canyons of
the range.
(a) Shore line retires permanently west of the valley border.
(6) Deposition of extensive fluviatile gravel beds up to elevations of 450 feet.
(c) Deepening of stream beds along valley border by 100 to 300 feet.
(d) Deposition of lower bench gravels.
(e) Deposition of present alluvium.
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THE SIERRA NEVADA.
TERTIARY RIVER GRAVELS AND VOLCANIC ROCKS.
Accumulations of gravel are found along almost all the Tertiary river channels now exposed
by erosion along the slope of the range. The only exceptions to this rule are the upper parts
of the stream courses near the present divide, where the grade evidently became too steep
for the accumulation of such detrital deposits. The areal extent of the gravels, as shown on a
geologic map, would not be large. Except those at a few places along the deep longitudinal
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