K. Bryan — Bock Tanks and Char cos. 193 



sediment carried. Streams of the size common in south- 

 west Arizona are competent to erode pools abont 10 to 

 20 feet in diameter and 3 to 10 feet deep. The shape of 

 the pool depends on the character of the rock and the 

 amount of modification due to joint-block plucking and 

 pothole formation. Plunge pools are the largest type 

 of channel irregularity, and consequently hold the largest 

 pools of water. Most important rock tanks are of this 

 class. 



Physiographic relations of rock tanks. 



From the foregoing discussion it is seen that the largest 

 rock tanks are plunge pools at the foot of falls, and that 

 even the other channel depressions are likely to be larger 

 and more important in the parts of a stream near falls 

 and rapids. The factors governing the occurrence of 

 falls are thus of importance. Falls are found in south- 

 west Arizona under at least three different circum- 

 stances: (1) where there are marked differences in the 

 ability of adjacent parts of the rock to resist erosion; 

 (2) where dissection of the mountain pediments on a 

 new grade produces headwater falls; (3) where renewed 

 uplift of fault-block mountains produces falls on a stream 

 crossing the fault line. 



Falls due to differing resistance of rocks to erosion. 



Falls wholly due to an unusually resistant rock were 

 found in only one locality in the Papago Country, and 

 on a minor scale ; but the locus of the falls due to other 

 causes is often determined by a resistant bed, as in the 

 case of the lower falls at Baker Tanks, southeast of 

 Wellton, Yuma County. The variety of circumstances 

 and the association of different types of tanks is well 

 illustrated by this watering place. The tanks are in a 

 streamway which leads in a northwesterly direction on 

 the southwest flank of the Baker Peaks. Below the 

 tanks the stream channel lies in a small canyon 150 feet 

 wide, and about 25 to 30 feet deep. Above the tanks 

 the channel is but 2 to 3 feet below the adjacent plains. 

 Between these two parts of the stream is a stretch of 

 falls and rapids about 1,000 feet long, which is shown in 

 iig. 3. The rock in which these falls occurs is a coarse, 

 arkosic conglomerate, which strikes N. 77° west, and 

 dips 65° southwest. The bulk of the material is a gra- 

 nitic debris with pebbles one-eighth to one-half inch in 



