158 SOUTHERN CALIFORNIA GEOLOGY 



berry fault lines above described and which are mere or 

 less connected structural depressions. Although recon- 

 noitered several times by the writer, little is as yet 

 known about them. Apparently the trough is a series 

 of down-sags between the fault lines mentioned. The 

 depth of sinking of the valley block and the fault dis- 

 placements on either side are relatively extensive. 

 Along the sides of the valley and out of its bottom sev- 

 eral well preserved volcanic craters rise, accompanied 

 by flows of ropy, basaltic lava as may be seen west 

 of Amboy and at other places.^ The latest of these 

 lava flows follow and fill drainage ways of the valley 

 floors. On this account they may be adjudged to be 

 quite modern. 



Extensive "dry" lakes also occur in the Amboy 

 trough, two of which are known as the Amboy and 

 Bristol Lakes. The former has a surface of gypsite 

 and is the source of much of the commercial gypsum 

 used in Los Angeles. Towards the southeast end of 

 that portion of the Amboy Trough which lies in Cali- 

 fornia, its Arizona continuation not having been 

 studied as yet, several small lost ranges occur. These 

 have northwest-southeast trends, and may be secon- 

 dary structural blocks. Near the Colorado River in 

 California the paths of these northwesterly extending 

 faults intersect with the faults of the east-west or 

 transverse belt, thereby adding further interest to the 

 geology of the region. 



THE OLD WOMAN FAULT ZONE 

 A zone of the northwest-southeast faulting passes 



'Some fifteen years ago the writer visited, studied and mapped some of 

 the phenomena of the Cadiz and Newberi-y faults and Amboy Trough and 

 adjacent ranges. 



