586 THE MIOCENE PERIOD 



been folded, and the folds overturned so that Cretaceous and Eocene 

 formations overlie the Miocene. 



Non-marine deposits. In the northern part of the central val- 

 ley of California there are deposits of estuarine, lacustrine, and 

 probably subaerial origin (lone formation) partly contemporaneous 

 with the early Miocene, marine beds farther south. They consist 

 of the common sorts of clastic sediments, with some coal, iron, etc. 



^^^^^ 



jpfMBffiJM 



Fig. 492. Section showing the structure and relations of the Miocene system in 

 the San Luis Obispo region of southern California. Jsl, San Luis formation, 

 Jurassic; Nm, Monterey shale, Miocene; Nrt, rhyolite tuff; Np, Pismo formation, 

 Miocene (?); Npr, Paso Robles formation, Pliocene; Pal, recent alluvium, etc. 



Along the east side of this valley, auriferous gravels, 1 brought down 

 by streams from the Sierras, were being deposited during at least a 

 part of the period. These gravels seem to have been laid down on a 

 surface of slight relief, interpreted as a peneplain. 2 The Sierra 

 Mountains are thought to have been at least 4,000 feet lower than 

 now when these gravels were deposited. 



Non-marine Miocene beds are rather widespread in south- 

 eastern California and Oregon, reaching great thicknesses at some 

 points in the vicinity of the 4oth parallel. They include clastic 

 sediments, volcanic debris, infusorial earths, and fresh-water lime- 

 stones. 



Farther east, on the western part of the Great Plains, the deposi- 

 tion of the White River beds may have continued for a time after the 

 beginning of the Miocene. Late in the period, aggradation seems to 

 have been renewed in the same general area, and the Loup Fork 

 formation, thin but extensive, was spread over great areas from 

 South Dakota to Mexico. The lacustrine phases of this formation 

 are probably less extensive than the subaerial. 3 Like the White 

 River formation, the Loup Fork beds have been eroded into "bad- 

 land" topography (Figs. 68 and 69). 



1 Turner, i4th Ann. Kept., U. S. Geol. Surv., 1894; Lindgren, Jour. Geol., 

 Vol. IV, 1896, pp. 881-906; Diller, Jour. Geol., Vol. II, pp. 32-54. See also folios 

 of the Gold Belt of Calif., U. S. Geol. Surv. 



2 Diller, Jour. Geol., Vol. II, pp. 33-54. 

 3 Haworth, Univ. Geol. Surv. of Kan., Vol. II, p. 281. 



