K. Bryan — Rock Tanks and Charcos. 



195 



the little gorge at the points marked A and B are pot- 

 holes abont 3 feet in diameter developed along joint 

 plains. Below the gorge is a flat abont 500 feet long, 

 which ends in a second fall. The flat has an irregular 

 rock floor, the hollows in which are covered with sand. 

 These hollows are dne to irregnlar erosion. At the point 

 marked C, there is a hollow somewhat deeper than the 

 others, in which water may usually be fonnd by digging 

 away the sand. 



The second or lower falls are abont 6 feet high, and 

 are caused by two beds of very coarse conglomerate 

 which resist erosion somewhat better than the rest of 

 the rock. The downstream bed of coarse conglomerate 

 has been largely cut through; the upstream bed has been 

 cut at only one point, near the abandoned pump. At 



Surface of 



/ int-ersrream area 



Clay gauge along 

 fault- zone 



J 



2ofeeh 



Fig. 4. Section along stream channel at Tabaseca Tank, Eiverside 

 County, Calif. 



this place two very perfect potholes about 10 feet in 

 diameter have been formed at one side of a narrow gully 

 in the rock. Both of these hold about 4 feet of water, 

 though the total depth of each is about 10 feet. Below 

 the falls is a plunge pool covered with sand, in which 

 water can be found by digging. 



The influence of a notably soft rock in the production 

 of falls is shown at Tabaseca Tank. 5 This tank lies in 

 an arroyo which drains the northern flank of the Choco- 

 late Mountains in sec. 31, T. 7 S., R. 14 E., M. D. B. and 

 M., Riverside County, California. The arroyo lies in a 

 narrow gully about 10 feet below the level of the sur- 

 rounding plain which is part of a slope cut on rock and 



5 Brown, John S. : The Salton Sink Eegion, a geographic, geologic, and 

 hydrologic reconnaissance, U. S. Geol. Survey Water-Supply Paper, in prep- 

 aration. 



