166 Adventures in Scenery 



close of Palaeozoic time, more than 200,000,000 years ago. 

 The roots of these ancient mountains are recognizable by the 

 character of the rocks and the fossils remains that are preserved 

 in them. This ancient mountain system was formed by the 

 uplifting and folding of a great series of layers of slate, shale, 

 and sandstone originally mud, silt, and sand, derived from 

 land which lay mostly to the west of the present border of the 

 continent laid down in an arm of the Pacific Ocean. These 

 beds aggregated thousands of feet in thickness. 



During the long lapse of time that followed the wrinkles 

 and folds in the earth's crust thus produced were in large part 

 worn away, and finally the region sank below sea level and 

 again became a place of deposition of sediments borne from 

 adjacent land surfaces. For millions of years layers of mud, 

 silt and sand, together with beds of volcanic material, accumu- 

 lated upon the submerged remnants of this earlier mountain 

 system, and then, at the end of the Jurassic period, about 

 130,000,000 million years ago, another upheaval occurred, and 

 the sediments which had been deposited upon the submerged 

 remnants of the former mountain system were folded and 

 crumpled, and were invaded by molten granite from below. 

 Thus there arose a second system of mountain ranges that occu- 

 pied most of eastern California, and indeed large areas in 

 adjoining States. Throughout the Cretaceous period, which 

 followed the Jurassic, this second mountain system was gradu- 

 ally worn down until by the beginning of the Tertiary period 

 only ridges and hills of moderate height were left. 



The present Sierra Nevada range was not formed until a 

 long time later. It assumed its present height and form about 

 the dawn of the Quaternary period. Throughout the preced- 

 ing Tertiary period, especially in the later half, the region was 

 the scene of repeated disturbances and minor mountain-build- 

 ing movements that finally led up to the culminating uplift 

 at the beginning of Quaternary. 



