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



nio and at Freestone they lie nearly horizontal and conform in 

 dip with the Sonoma Tuff. Just prior to the Merced period 

 extensive volcanic disturbances took place, probably having their 

 center somewhere northeast of Santa Rosa Valley. Among the 

 reasons for this belief as to the locality of the center of disturb- 

 ance are the position of the overturned redwoods of the Petrified 

 Forest, with their roots pointed to the northeast, and the great 

 thickness of the tuffs and lava flows in their vicinity. 



The volcanic disturbance was very great, and continued 

 throughout the whole of the Merced. The first outflows con- 

 sisted of pyroxene-andesite, which in the neighborhood of Mt. 

 St. Helena aggregates a thickness of about 1,500 feet. Follow- 

 ing this came a great outthrow of andesitic pumice and lapilli, 

 with occasional thin flows of basalt, which spread over the 

 country to a maximum depth of nearly 2,000 feet, and were car- 

 ried down the streams into the lakes and seas, forming beds of 

 tuff and conglomerate from ten to two hundred feet thick. 



While the volcanoes were throwing out this vast amount of 

 ashes and fragmental material, deposits, other than the tuff, were 

 forming in the seas and lakes, particularly in the region to the 

 west. Buff colored sandstone, and fine conglomerate with 

 numerous volcanic pebbles similar to those of the tuff, were 

 deposited in the region of Santa Rosa Valley to the depth of 

 between 2,000 and 3,000 feet. In the upper part of these beds 

 (Wilson Ranch Beds) fossils of Merced age were preserved. 

 Lakes of great size probably existed, since the relation of the 

 upper lava flows, the St. Helena Rhyolite, to Becker's Cache 

 Lake Beds, and the correlation of the latter by Marsh with the 

 late Pliocene, point to a large Merced lake in that neighborhood. 



While the sediments were depositing in the seas and lakes, 

 large rivers were forming deltas with coarse gravels to great 

 depths, thus showing that a gradual subsidence was taking place. 

 A good illustration of this may be seen between Mark West 

 Springs and Santa Rosa Valley, where at least 2,000 feet of 

 coarse gravel has accumulated. The history of this old delta 

 bears a striking similarity to that of the Santa Clara-San Benito 

 Valley.* 



*The Post-Pliocene Diastrophism of the Coast of Southern California. 

 Lawson. Bull. Dep. Geo!., Univ. of Cal., Vol. 1, No. 4, p. 151. 



