386 



University of California. 



[Vol. i. 



altogether beneath the sea-level," and take up its history at a point 

 subsequent to that time. 



The occurrence of a great orogenic disturbance at the close of 

 the Miocene, giving birth to the Coast Ranges as a connected moun- 

 tain chain, appears to be one of the best established facts in the geol- 

 ogy of California. From that disturbance dates the history of the 

 Great Valley. During Pliocene times the Coast Ranges, although 

 depressed from 800 to 2,100 feet below their present level,* still 

 persisted as a more or less continuous land barrier, as shown by the 

 development of extensive peneplains with accompanying delta-for- 

 mations, and the local accumulation of great thicknesses of littoral 

 Pliocene sediments which rest unconformably upon the Miocene. 



According to the U. S. Geological Survey ,f the Great Valley 

 was probably occupied during the whole of the Neocene by a gulf, 

 connecting with the ocean by one or more sounds across the Coast 

 Ranges, — which is in general harmony with the foregoing. As yet, 

 however, we can say very little about Pliocene sediments in the % 

 Great Valley. The geologists of the Geological Survey, working 

 on the northern and eastern portions of the valley, have found it 

 impracticable to subdivide the Neocene, and in general, it may be 

 said that we are without any authoritative information concerning 

 marine Pliocene deposits within the Great Valley, although they 

 are found at Kirker's Pass, high up on its western rim. J Their ap- 

 parent absence would indicate that the western barrier was either 

 more continuous than would appear to be the case from a study of 

 the Coast Ranges themselves, or that the present floor of the valley 

 was above sea-level, in which case it must have been a gently slop- 

 ing plain rather than a valley, or, lastly, the rivers draining the land 

 to the east may have been so augmented in flow as to prevent, by 

 the influx of a large volume of fresh water and the deposition of 

 gravels in their turbulent currents, the laying down of sediments 

 bearing marine organisms. It is probable that no one of these three 

 alternatives obtained to the exclusion of the other two. In spite of 



*Lawson, Bull. Dept. Geol. Univ. Calif., Vol. 1, pp. 157 and 270. 

 t Description of the Gold Belt, Jackson Folio, etc., U. S. Geol. Survey. 

 % Turner, Geology of Mt. Diablo, Bull. Geol. Soc. Am., Vol. II, p. 396, 

 with references to Whitney and Gabb. 



