The Valley of the South 259 



folding and crumpling of the rocks occurred. The softer clays 

 and sands south of the San Bernardino Valley yielded to earth 

 compression and were folded and crumpled. The Perris plain 

 was uplifted somewhat, but more in the southern part. The 

 faulted block comprising the Perris plain was rotated toward 

 the north, that is, it was elevated in its southern portion and 

 tipped down at the north. The granite which forms the floor 

 of the Perris plain being more rigid resisted folding and moved 

 as a block. 



Fold in Rocks Forms Dam Across Valley 



A wrinkle or fold that has great significance in the San Ber- 

 nardino Valley extends from the San Jacinto Mountains north- 

 westward along the line of the Badlands between the San 

 Timoteo Canyon and the San Jacinto Valley. The rocks that 



Photo by W. C. Uendenhall, U. S. Geol. Survey 

 FIG. 75. Bunker Hill Dike. Not a dike but a fold. Near San Bernardino. 



were folded into this arch are soft shales, sandstones, and grav- 

 elly alluvium, much like that deposited by rivers today in the 

 San Bernardino Valley. This is known as the Bunker Hill 

 Dike (though it is not in any proper sense a dike) . This clay 

 and gravel ridge forms an effectual dam across the valley behind 

 which waters percolating through the stream wash accumulated 

 in the upper basin are held until they rise as springs and flow 



