536 STUDIES FOR STUDENTS 



Playas will be flooded with water partly by means of stream 

 channels, partly by a general wash from the outside over the slopes, 

 partly by a gradual rise of the water level flooding the surface from 

 the inside. In places of inflowing currents, the cracks should fill 

 up with sand and thereby permanently preserve the structure; in 

 places where clay settles in from quiet water, the infiltration may 

 be almost identically the character of the wall materials and the 

 former presence of the crack therefore escape record. The water 

 stands over the flat bottom in periods varying from a few days to a 

 few years, according as to whether it is an evanescent playa or one 

 dry only during an occasional year. The periods of desiccation 

 will vary in inverse order. In general, however, it may be said 

 that the playa bottoms become thoroughly wet for a depth of several 

 feet and undergo some months of desiccation with the formation 

 of deep mud -cracks. Where the deposits are perfectly homogeneous 

 and result in massive saline clays there may be no permanent record 

 of the cracks. As playas are characteristic of typically desert regions 

 there is but little likelihood of the incorporation of an organic record^ 

 either of leaves, bones, or tracks. As embodied in the geological 

 record playas should occupy the centers of flat basins in mature 

 desert regions, and in that case their deposits may conceivably attain 

 a thickness of several thousand feet as the mountains are gradually 

 leveled off and their waste accumulated in the tectonic intermontane 

 troughs. Such deposits as seen in cross-section would pass irregu- 

 larly into marginal waste slopes of coarser material and these in 

 turn end unconformably against the sloping walls of the buried 

 portion of the mountains. Thus their basin nature and limited 

 extent would be evident. 



Unless protected, however, by an invasion of the sea or a change 

 to a pluvial climate such deposits as well as the intermediate rocky 

 barriers will gradually be removed by deflation, as Passarge has 

 shown, ^ and the desert will pass into the stages of old age as exempli- 

 fied by the Kalihari. Throughout this process of erosion shallow 

 playas play an important role, since the occasional rains wash the 

 surrounding waste into them and thus tend to maintain a level 



I "RumpfHachen und Inselberg, " Zeitsch. deut. geol. Gesellsch.. LVI (1904), 

 Protokol., pp. 193-209. Review by W. M. Davis, Science, N. S., Vol.XXI, p. 825. 



