K. Bryan — Rock Tanks and Char cos, 191 



Most rocks are divided into blocks by sets of inter- 

 secting fractures or joints. The impact of the current 

 and of sediment which it carries tends to break out 

 blocks of rock in the stream channel. This process of 

 plucking 1 results in irregularities of the stream channel, 

 as shown in fig. 1, a. When the joints are widely spaced 

 the blocks are large, and the depressions are correspond- 

 ingly large. Closely-spaced joints, however, produce a 

 rough but comparatively even-floored channel. It is 

 obvious that very wide spacing of joints will produce joint 

 fragments too large to be easily removed and that there 

 is for any stream an optimum spacing of the joints which 

 will produce the maximum roughness of channel floor. 

 In these rugosities water remains after a flood, but these 

 pools are seldom of great importance as watering places. 

 Commonly the process of joint-block plucking is com- 

 bined with the processes detailed in the following para- 

 graphs. A favorable spacing of joints is essential for 

 the production of the larger rock tanks. The fractures 

 must also be water-tight or nearly so in the bottom and 

 downstream wall of the tank, else the water will drain 

 rapidly. In some tanks it is evident that joints on the 

 upper side of the cavity are open, and through them 

 small amounts of water seep into the tank for short 

 periods after floods. 



Where the channel of a stream is curved the swiftest 

 thread of the current is near the outside of the bend. 

 The maximum erosive force of the current is exerted over 

 a crescentric area in the bend, as is shown in fig. 1, b. 

 These areas are likely to be scoured below the grade of the 

 stream, producing the hollows here called scour depres- 

 sions. In combination with joint-block plucking and the 

 formation of potholes scour depressions are likely to 

 form important tanks in hard rock. A scour depression 

 in relatively soft hardpan appears to be the cause of 

 one of the waterholes in the Vekol Valley, Maricopa 

 County, 1ig. 2. This consists of a hollow at ^he point of 

 an elbow in the sandy channel of the stream. A cavity 

 in the underlying hardpan holds water after a flood. The 

 pool of water is 6 feet by 4 feet, and 2 feet deep, located 

 under the bank. Seepage from the sand of the stream 

 channel and from the sandy clay of the banks maintains 

 the water for a considerable time after adjacent charcos 

 have dried up. The water does not last very long, for 

 the level fell iy 2 feet between September 20 and Sep- 

 tember 22, 1917. 



