K. Bryan — Rock Tanks and Charcos. 205 



are depressions below the general level of the adobe 

 flats and in them water may remain after the passage of 

 the flood, fig. 12. Whether water does so remain depends 

 upon the porosity of the bottom of the channel. If 

 erosion has carried the bottom of the channel below the 

 adobe to some bed of sand or fine gravel, water will 

 easily drain away underground. If, however, the chan- 

 nel is entirely in the usual fine-grained, claylike adobe, 

 water will seep away more slowly. The principal losses 

 will then be from evaporation or, dependent on circum- 

 stances, from use by stock and wild animals. 



Fig. 12. 



In this fashion the larger charcos are formed. The 

 smaller ones, however, usually occur along some rela- 

 tively small stream which spreads out over part of its 

 course in a small, often grassy, flat. These flats are often 

 only a few feet wide and a few yards long. Channel cut- 

 ting of the kind previously described sometimes takes 

 place in these flats, and many of the smaller charcos seem 

 to be due to a breaking of the grass cover which allows 

 erosion to take place over a very small area. Other holes 

 seem to be due almost wholly to the activities of animals, 

 both wild and domestic, which come to feed in the flat 

 immediately after the rain. Very shallow pools of water 

 attract them; they drink the muddy water, roll in the 

 mud, and trample and compact the bottom. Thus a 

 somewhat deeper hole is formed which, when the next 



Am. Jour. Sci.— Fourth Series, Vol. L, No. 297.— September, 1920. 

 15 



