NEOCENE STRATIGRAPHY. 35I 



beds upon which they lie. But though so similar in ap- 

 pearance, erosion discloses a marked difference in the 

 solidity of the two formations. In the deposit at the head 

 of Wood's Gulch, already described, the end of a tusk, 

 presumably of elephant or mastodon, was found about 75 

 feet below the top of the deposit. 



Though these deposits for the most part appear to be 

 of fresh water and wind origin, there are some facts that 

 suggest that they are, in part, at least, of marine origin. 

 In some cases the top of the deposit is so little below the 

 elevation of the surrounding land, its area so large, as 

 compared with the area of the hills from which it might 

 derive material, that we are led to suppose, either, that the 

 surrounding points are almost reduced to base level (in this 

 case the top of the deposit), or else that submergence has 

 permitted the deposition of marine deposits. There is 

 one feature that strongly favors the latter theory. Nearly 

 all the streams running to the bay show a marked terrace 

 almost to their head. In most cases the streams have cut 

 down through this terrace, revealing deposits from a few 

 feet to twenty-five or thirty feet in depth. In many cases 

 these cuts are very recent, as traces of wagon roads are 

 still visible at their heads, or are shown upon the U. S. 

 Coast and Geodetic Survey map of 1869. These terraces 

 follow about the present inclination of the stream bed, 

 and, though they can seldom be traced continuously, 

 would appear to join the general level land which slopes 

 from the foothills to the bay. The character of these 

 stream terraces, like the isolated deposits described in the 

 preceding paragraph, is varied. In places they appear 

 to be beyond a doubt subaerial stream deposits, containing 

 trunks of spruce or redwoods, sometimes three or four 

 feet in diameter. The wood in these trees is still in good 

 preservation. Above the trees, which lie horizontally, 



