74 Adventures in Scenery 



sand, together with beds of volcanic material, accumulated 

 upon the submerged remnants of the ancient mountain system. 



At the end of the Jurassic period, about 130 million years 

 ago, there came another upheaval and the new sediments which 

 had been deposited on the sea bottom were upheaved and became 

 dry land. These rocks were folded and crumpled, and invaded 

 by molten granite from below. Thus there arose a second sys- 

 tem of mountain ranges that occupied most of eastern Cali- 

 fornia and the site of the present Sierra Nevada Range. 

 Throughout the Cretaceous period, which followed the Jurassic, 

 this second mountain system was gradually worn down, until 

 by the beginning of Tertiary time only ridges of moderate 

 height were left. 



The degradation and removal of the Franciscan formations 

 was interrupted by the depression or subsidence over a wide 

 area that ushered in the Cretaceous period. With the beginning 

 of Cretaceous much of the Coast Range region again became 

 submerged. The Cretaceous formations were once co-extensive 

 with the territory now occupied by the Coast Ranges. Al- 

 though removed by erosion over wide areas, where Franciscan 

 and other rocks now appear at the surface, they still constitute 

 one of the largest elements in the stratigraphy of the region. 

 They are composed chiefly of shales and sandstones. 



The Franciscan rocks have been much broken, folded, and 

 crumpled by earth eruptions. Dikes of molten rock were 

 forced up from below. Lavas also were poured out upon the 

 surface. During the long interval while the Franciscan rocks 

 remained above sea level they were worn and eroded and the 

 surface was reduced to a peneplain. This means that the moun- 

 tains that had been upheaved were worn down by the long- 

 continued weathering of the elements and the erosion of streams 

 till they became low hills and ridges. The subsidence of a great 

 area transformed this landscape from an undulating and deeply 

 eroded plain to a vast flat sea bottom on which sediments from 

 the adjoining land areas were deposited. This time of erosion, 



