K. Bryan — Rock Tanks and Char cos. 189 



irregular erosion of rocks, and are usually small. They 

 are mere pockets in the rocks which hold water for a 

 few hours or days after a rain. The rock tanks formed 

 in stream channels are mostly plunge pools and pot- 

 holes associated with falls. In the Papago Country they 

 seldom exceed 10 feet in diameter and 5 feet deep. Sand 

 tanks are similar basins which are filled with coarse 

 sand by the tail end of floods. Water is held in the 

 interstices of the sand. Since rock tanks are generally 

 associated with falls, the distribution of these is of 

 prime importance. In the Papago Country falls occur 

 in streams mainly because of: (1) renewed uplift along 

 faults parallel to the mountain ranges, but at right 

 angles to most of the mountain canyons; (2) dissec- 

 tion of rock plains or pediments at the foot of moun- 

 tains by headward cutting of small rock gorges. Falls 

 produced by either cause, and therefore most rock tanks, 

 occur at or near the borders of the mountains. 



Charcos are depressions in adobe flats over which 

 floods spread widely. The important charcos are 

 formed by the swiftest thread of the current, which ex- 

 cavates a channel deeper in some places than in others. 

 After floods, pools of water remain, the larger of which 

 are from 15 to 30 feet across, and from 20 to more 

 than 1,000 feet long. The largest adobe flats, and there- 

 fore most of the charcos, lie at the centers of the inter- 

 montane valleys. 



Rock Tanks. 



Definition. 



A rock tank is a cavity or depression in rock which is 

 filled periodically with rain or flood water. The Mexi- 

 cans commonly, and many Americans, use the Spanish 

 word "tinaja," a bowl or jar, in speaking of rock tanks. 

 These cavities may occur either: (1) away from stream 

 channels, or (2) in stream channels. 



Bock tanks away from stream channels. 



In mountains or hills small rock pockets are found 

 which are due to the unequal weathering of rock surfaces. 

 They vary in size from a few inches across and a half- 

 inch deep to pans which are from 5 to 20 feet across 

 and 6 to 8 inches deep. Such pockets hold water for 

 longer or shorter periods following a rain and are of 

 little value as water supplies. Experienced hunters and 



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



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