GEOLOGY OF THE SIERRA NEVADA. 51 
formations to be described in the next chapter. These are the marine 
sedimentary strata found in the foot-hills at numerous points all along 
the eastern margin of the Great Valley. These marine deposits, like those 
higher up in the mountains, rest unconformably on the upturned edges of 
the auriferous slates. They are usually well supplied with fossils, so that 
there can be no doubt as to their geological position, or the nature of the 
medium in which they were deposited. They include strata both of Tertiary 
and Cretaceous age ; and, although there are localities where they might be 
confounded with the true auriferous gravel deposits of fresh-water origin, 
there is usually no difficulty in distinguishing the two formations, as will be 
evident from the following brief sketch of the character and mode of occur- 
rence of the marine strata. 
The greatest development of these deposits is in the southern part of the 
Great Valley, and especially along the foot-hills between Kern and White 
rivers. Here this formation is from 200 to 600 feet in thickness, and it is 
either horizontal or has a slight dip to the east, away from the mountains, 
from the erosion of which its material appears to have been derived. The 
rock is usually a soft sandstone, chiefly made up of granitic débris. From 
Kern to King’s River the older metamorphic rocks come down to the edge 
of the valley, and are there covered by recent detrital beds. Farther north, 
and extending to the Stanislaus, there is a belt of low flat-topped hills of 
sandstone, very much eroded, so as to leave many little isolated patches, of 
from a few feet up to 150 in elevation. All these deposits to the south 
of the American River appear to be of Tertiary age, and a portion of them 
were identified by Mr. Gabb as Miocene. The upper portion of some of these 
Tertiary hills are of fresh-water origin, containing bones of land animals and 
fragments of wood. There are also localities where the volcanic detrital 
material has found its way entirely down to the level of the Great Valley, — 
an occurrence which will be easily understood, after the description of the 
auriferous gravels and associated volcanic materials in the next chapter has 
been consulted. 
On the American River, just below the town of Folsom, the Cretaceous 
formation makes its appearance, and there are other small patches of the 
same, well filled with fossils, in the vicinity. Farther north the Cretaceous 
occupies a considerable area, the best exposures being found to the north of 
Oroville. This formation is cut through in the foot-hills by the streams 
coming down the slope of the Sierra, and there are excellent exposures of 1t 
