The Geological Story Briefly Told 63 



nent very widely occurred. This was a time of mountain- 

 making on a grand scale. It affected the eastern half of the 

 continent, however, more than the western. The Rocky Moun- 

 tains had not yet been born, so to speak. The general outline 

 of the continent was much as it is now. Much of the western 

 half of the continent was covered by the sea and was receiving 

 sediments from the land areas. There were lands above sea level 

 where the Rockies now are, but this great system of mountains 

 had not yet been uplifted. The first ancestral mountains where 

 is now the great Sierra Nevada Range date from the Permian 

 period, some 200 million years ago. The Permian was the 

 transition period between the Palaeozoic and Mesozoic eras, or 

 between the Carboniferous and the Triassic periods. It was 

 during Permian time that the first definitely known upheaval 

 occurred of what is now the Sierra Nevada Range. 



The Basement Complex 



How far back in geologic history the landscape of California 

 reaches we do not know definitely. The most ancient sedi- 

 mentary formation, that which rests upon the upheaved, con- 

 torted, crushed and broken "basement complex," as the under- 

 lying granitic bed-rock is called, is the Franciscan (Jurassic). 

 These rocks occur over a wide area, extending both north and 

 south of San Francico Bay, and hence the name. Rocks of the 

 Franciscan formation, or perhaps it would be more correct to 

 say of the group of Franciscan formations, rest unconformably 

 upon the older rocks below. "Unconformably" is a big word, 

 but it has a definite meaning in geological literature, and because 

 there is no better or simpler word we are compelled to use it. 

 It means that there is a break in the record of the rocks, a lost 

 interval, a hiatus or gap in the succession of formations. When 

 we say that the Franciscan rocks rest unconformably upon the 

 older rocks below this means that there was a land surface long 

 exposed to erosion and the weathering of the elements, and that 

 later this old land surface was depressed so that it was covered 



