Mountains of the South 237 



The Glacial Period in geologic history is a part of the 

 Quaternary Era, which is the era just preceding the Recent, in 

 which we live. The history of the San Bernardino Range dates 

 back to pre-Cambrian time, which marks the earliest stages of 

 the history of the earth. The Cambrian formation marks the 

 beginning of geologic history, as revealed in the sedimentary 

 rocks in which fossil remains of living things furnish the key 

 to the successive stages of progress. In pre-Cambrian time, 

 that is, some time during the indefinitely long time before the 

 oldest fossil-bearing sedimentary rocks were deposited, molten 

 rock was forced up from the depths of the earth under the 

 region where the San Bernardino Mountains now are. These 

 once molten rocks uncovered by erosion now appear as crystal- 

 line granitic rocks. 



History of San Bernardino Range 

 Is Long 



The details of the geologic story are long. The region of 

 the San Bernardino Mountains has long been land, and it has 

 at times been covered by the sea. This means that the region 

 has been elevated, and has in turn subsided or been depressed or 

 sunken. That it has been rent by earthquakes and the rocks 

 broken is shown by the faults or breaks in the crust of the earth, 

 such as the San Andreas fault. 



That the region was below sea level is shown by the forma- 

 tions of sandstone, shale, and limestone that are now part of 

 the rocks of the range. These sediments have been intruded, 

 crushed, and folded by later intrusion of molten rock from 

 below. Heat and pressure from the molten materials changed 

 (metamorphosed) the rocks with which they came in contact. 

 Sandstones became quartzites, limestones became marble, shales 

 and other rocks became schists. Thus the rocks that are now 

 seen in the San Bernardino Mountains are very complex. 



In comparatively late geologic time (technically late Ter- 

 tiary) the region was uplifted, and this means that erosion of 



