Vol. 4 1 Osmont. — Geological Section of Coast Ranges. 



upon a wave-cut shelf just about at high-water mark and extend- 

 ing up to 113 feet above the same, consisting principally of dio- 

 rite sands, and occasional pebbles showing very indistinct hori- 

 zontal stratification and cross-bedding. On the ocean side of 

 the peninsula occasional still smaller patches, some twenty to 

 thirty feet thick, may be seen resting upon a very evenly worn 

 diorite surface, which at a point about three miles south of the 

 mouth of Salmon Creek dips gently toward the north and passes 

 under the beach and aeolian sands. On the eastern side of the 

 bay is a broad, flat terrace, about one-quarter of a mile in width 

 and some seventy -five to ninety feet above sea level at its back. 

 In most places only a thin veneer of gravel covers this terrace, 

 but at one point on shore, at the north end of the bay, a remnant 

 of gravel some fifty feet thick is resting upon the worn Fran- 

 ciscan sxirface, which is here only some twenty feet above sea 

 level. It is made up chiefly of Franciscan pebbles, and loosely 

 coherent sands showing cross-bedding. It has been somewhat 

 distorted, dipping slightly to the north. 



Flood Plain Deposits. — All the larger valleys of this portion 

 of the Coast Ranges, such as Santa Rosa, Sonoma and Napa Val- 

 ley, and the great Sacramento Valley itself, show extensive flood- 

 plain deposits extending up to an elevation many feet above the 

 present level of the streams. These terraces are usually covered 

 with, or largely made up of, ill-assorted heterogeneous material, 

 representing all the rocks of the immediate neighborhood. In 

 the ease of the large streams like the Russian River, nearly all 

 the rocks of the whole area described in this paper are repre- 

 sented. Usually the stratification is very irregular and indis- 

 tinct, cross-bedding being common. Good exposures may be seen 

 along the railroad cuttings at many places in Santa Rosa Valley. 

 Here the material is usually made up of a very light colored, 

 almost white, incoherent, silt-like sand, with much moderately 

 fine gravel containing a great variety of well worn pebbles, con- 

 sisting of Franciscan cherts, quartz, sandstones and intrusives, 

 Knoxville sandstone, and a large proportion of Tertiary volcan- 

 ies, conspicuous among which is a white rhyolite. 



