206 "jr. Bryan — EocA; Tanks and Charcos. 



rain comes, will hold water for a longer time, and thereby 

 undergo the same processes for a longer period. The 

 maximum depth below the surface attained by this proc- 

 ess seems to be about two feet. 



In South Africa Passarge has found large basins which 

 seem to be wholly due to the ^activities of animals. He 

 has called the process zoogenous erosion. The animals 

 involved are rhinoceri, elephants, buffalo, and wild hogs, 

 whose size and weight being greater than that of Arizona 

 animals, probably make possible the greater depth and 

 size of the African basins. 8 



The larger charcos are, however, the only ones of great 

 importance to travelers, because only these hold sufficient 

 water to last for more than a few days after a rain, or 

 are relatively permanent in position from year to year. 

 Generally, even the larger charco is never quite full, hold- 

 ing not more than 3 or 4 feet of water. When one of 

 these large charcos has once been formed it tends to be 

 relatively long-lived, for the trampling of animals pro- 

 tects its bottom from seepage, in the same fashion that 

 the bottom of reservoirs is puddled, and this trampling 

 also increases the resistance to erosion of the bed of the 

 charco. The trails which animals make in going to and 

 from water are often from 3 inches to a foot deep, and 

 tend to turn water toward the charco. In this connection 

 note the trails between the bushes in fig. 12. However, 

 a change in the main current of the flood waters may leave 

 the charco on the edge of the floods, in which case it is 

 very likely to become gradually filled up and obliterated. 

 There are certain localities where a charco will be recur- 

 rent because floods are normally constricted in this part 

 of their course. Such places are commonly along the 

 foot of some isolated hill in an alluvial plain near the 

 mainflood channel. A somewhat similar rocky projection 

 of the wall of a valley will produce the same result. If 

 for any reason the charco is temporarily filled up, a new 

 one will form within a short distance. 



Still other charcos are formed by the natural damming 

 of the channel in an adobe flat. The mudhole near the 

 Brush Corral in the Vekol Valley is due to the damming 

 of the narrow channel of the main drain of the valley, at 

 the lower end of a grassy flat by coarse gravel and sand 

 brought in by a tributary stream. The lower end of this 

 narrow channel remains full of muddy water after a 

 flood and forms a charco. 



8 Passarge, Siegfried: Die Kalahari, Berlin, 1904, p. 660. 



