

4 



THE AUKIFEEOUS GRAVELS OF THE SIEIIEA NEVADA. 





the aspect of a narrow lake. Preference will be made farther on to some of 

 these points, in connection with remarks on the profile of the former and 

 present river grades in the gravel region. 



The drainage into the Great Valley is very peculiar in character, and 

 is the combined result of the climatic and topographical conditions of the 

 region. This will be explained after a brief description of the chains of 

 mountains which form the framework of the valley. 



The Coast Ranges form the limit of the Great Valley on the western side, 

 and extend to the Pacific Ocean, there being nowhere on that side more than 

 a narrow space between the foot-hills and the ocean, while for a large part 

 of the distance the steep slopes of the ranges come directly to the water's edge. 

 A glance at the "Map of California and Nevada" will show in a few moments, 

 much better than could be explained in many words, the peculiar character 

 of the topography of the Coast Ranges. The inosculation of the coast moun- 

 tains with the Sierra Nevada at both ends of the Great Valley has already 

 been alluded to. The general fact is at once recognized that the Coast 

 Ranges are made up of numerous broken and often rather indistinct chains, 

 which on the whole maintain a pretty well marked parallelism with the 

 coast. This parallelism, however, is often better made out from an examina- 

 tion of the courses of the rivers than from the position of the subordinate 



It will also be seen on the map, that while the coast mountains are 

 often nearly broken through by cross fractures, giving a chance of escape for 

 the secondary drainage, — 



ranges. 



that is, for the streams originating within the 

 Coast Ranges themselves, — in only one place does the fracture or depression 

 extend entirely across the whole series of chains. This takes place at the 

 Bay of San Francisco, where is the only outlet for the entire drainage of the 

 Great Valley. Here, in latitude 38°, the Sacramento and San Joaquin rivers 

 unite in an extensive depression, partly occupied by low islands covered 

 with a dense growth of "tule" (Scirpus palcstris), and subject to overflow 

 where not artificially protected, and partly by Suisun Bay and the Bays of 

 San Francisco and San Pablo. Of these bays the two latter are in reality 

 portions of one and the same thing, there being a narrow strait separating 

 them, and they form a depression about fifty miles long, lying parallel with 

 the general trend of the ranges, and enclosed within them ; while Suisun 

 Bay, on the other hand, is rather at right angles to the others, trending across 

 the ranges, and having no streams entering it, except the Sacramento and 

 San Joaquin. It is, in fact, the half-submerged delta of these two rivers, and 







