208 SOUTHERN CALIFORNIA (tKOLOGV 



SOUTHERN CALIFORNIA BEFORE THE 

 GLACIAL PERIOD 



The geographic conditions in Southern California 

 during the Pliocene prior to the mountain-making 

 revolution near its close, which initiated the Pleistocene 

 epoch are not satisfactorily known, but were approx- 

 imately somewhat as follows: 



A background of the older lands existed, consisting 

 of the Basin Range Region, some Coast Ranges, a 

 Sierra Nevada highland less conspicuous than the one 

 of today and possibly a diversified lowland area, frag- 

 ments of which now occur as the summit lands of 

 the San Bernardino, San Gabriel and Peninsula High- 

 lands. The Gulf of California Depression extended 

 even farther northward than today into the present 

 site of San Gorgonio Pass. 



The land area mentioned constituted a nearer sea- 

 level country than now, with its rivers and fresh- 

 water lakes and low hills and ranges. Its shore line 

 did not extend seaward as far as it does today. The 

 site of Los Angeles was then under water. 

 THE PLIOCENE PENEPLAIN 



This hypothetical lowland, developed at or near 

 the close of the Pliocene, constituted the stage-setting 

 for all of the great events which were to take place 

 in Southern California during the succeeding Pleisto- 

 cene and Recent Epochs, when it was sliced into fault- 

 block segments by the great rifts we have described, 

 and its fragments elevated or depressed into the 

 master highlands and valleys, practically^ as we see 

 them today. By this means the physiographic environ- 

 ment upon which all other conditions were to depend — 

 climatic, scenic and cultural — was developed. 



