196 



K. Bryan — Rock Tanks and Char cos. 



extending along the front of the mountains. The plain 

 is underlain by rhyolite which, in the vicinity of the tank. 

 is broken along a fault plane that crosses the stream and 

 dips upstream about 25 degrees (fig. 4). Shearing and 

 decomposition by water percolating along the fault plane 

 have changed the originally hard rhyolite to soft, claylike 

 gouge about one foot thick. The gouge of the fault in 

 its resistance to stream action is similar to any other 

 soft rock layer. As the stream cuts down its bed the tri- 

 angular block on the upstream side is constantly under- 

 mined by the ease with which the clay gouge is eroded. 

 With the waterfall once initiated, the formation of a 

 plunge pool is made easy by the presence of the soft 

 layer, and the channel is excavated below grade. The 

 pool is about 20 feet in diameter and filled with gravel 



Alluvial Slope 



• Dissected Mour>|-air> Slopes 



Pedinaepl- 



Fig. 5. Diagram showing the production of falls and tanks by erosion of 

 mountain pediment on a new grade. 



and bowlders. Water is found in the gravel throughout 

 the year unless campers have drawn too heavily on it. 

 Falls due to changes in stream grade. 

 Many of the mountains of southwest Arizona are sur- 

 rounded by plains, sloping to the intermontane valleys. 

 These plains or pediments 6 are underlain by hard rocks 

 similar to those of the mountains. The streams which 

 once wandered more or less at will are in most of the 

 pediments now entrenched in steep-walled gullies or 

 little canyons which are deeper toward the mountains. 

 The change in grade does not appear to be due to uplift 



6 Bryan, Kirk : Mountain pediments ; a discussion of the erosion of desert 

 ranges. Paper delivered Dec. 31, 1919, before the Geological Society of 

 America, Boston, Mass., to be published under the title: Geology and Physi- 

 ography of the Papago Country, Ariz., by the U. S. Geol. Survey. 



