K. Bryan — Rock Tanks and Char cos. 197 



and seems to be due to a change in the supply of sedi- 

 ment and the volume of the streams. This change is 

 perhaps climatic, and has certainly been general over 

 large parts of southern Arizona and northern Mexico. 

 The little canyons of the pediment increase in length by 

 headward erosion. At the head of each little canyon is a 

 falls or rapids which marks the separation between the 

 old grade and the new. These relations are brought out 

 in the diagram, fig. 5. 



Since each stream which heads in the mountains suf- 

 fers such a change in grade there are many falls. 

 Though not every falls produces a plunge pool of sufficient 

 size to make an effective watering place, the prevalence 

 of falls due to the dissection of the mountain pedi- 

 ment is the principal reason for the large number of rock 

 tanks in the desert region. The local conditions at each 

 falls determine the size and effectiveness, as watering 

 places, of the plunge pool and associated potholes. The 

 variety of these circumstances can best be illustrated by 

 an example. 



Black Tanks is a well-known watering place in the 

 Crater Mountains, Pima County. The tanks consist of a 

 series of plunge pools numbered from 1 to 8, in fig. 6, with 

 two very temporary waterholes marked 9 and 10. The 

 stream has a sandy bed about 2 feet below its flood plain 

 in the open valley above and west of the tanks. Below 

 the falls section with its 8 tanks, the stream flows in a 

 steep-walled gully with banks from 10 to 20 feet high. 

 The falls and rapids extend about 500 feet, with a total 

 drop of about 50 feet between the upper and lower chan- 

 nels of the stream. Throughout this stretch the stream 

 runs in a rocky gorge of variable width. The individual 

 falls are determined by the resistance of the horizontal 

 layers of lava which form the bedrock. The larger 

 plunge pools are from 4 to 5 feet deep and 10 to 15 feet 

 wide. 



On either side of the channel is a rock bench which 

 varies in width from 50 feet to several hundred feet. 

 This bench slopes gently upward to the foot of the steep 

 slopes of the hills, fig. 6, inset cross-section. On the 

 north side of the stream below the tanks the bench is cov- 

 ered with from 2 to 3 feet of cemented gravel. The rock 

 benches extend around the hills and are continuous with 

 similar rock benches which connect with the mountain 

 pediment. It is evident that Black Tanks are the head- 



