6o4 PRINCIPLES OF STRATIGRAPHY 



the salina. As already noted, the thickness which such a salt 

 deposit may reach is practically limited only by the depth of the 

 basin and the supply of the salt. 



The deeply cleft surface of the dry playa is not infrequently 

 buried by the wandering sands of the desert, while the rifts between 

 the polygonal blocks are filled with wind-blown material, or the 

 mud of the next succeeding inundation, and so preserved. This is 

 made possible by the rapidity with which the playa surface becomes 

 flooded, a case being on record where a lake lo to 15 kilometers 

 wide and of immeasurable length, though only from an inch and 

 a half to a foot in depth, came into existence in twenty minutes. 

 (Obrutschew, quoted by Walther-57 ://o. ) It must, however, be 

 noted that the dried surface of the playa is not infrequently 

 softened on being wetted, and tliat from swelling and flowage of 

 the mud the cracks may be closed again before they are filled. In 

 this manner many mud-cracked surfaces are again obliterated. 

 From preliminary experiments, Barrell concludes (2:557-5^^) that 

 "a mud-cracked loam or silty clay, even when the sand particles 

 are imperceptible to the fingers, is an unfavorable material for the 

 preservation of its detailed surface features, . . . Upon being 

 wet by rain the rapid swelling and disintegration of the surface 

 stratum would turn the surface of such a deposit into a creamy 

 mud. . . ." On the other hand, "a pure clay, slowly subsiding 

 from quiet waters, and wet sufficiently long to become compact 

 upon drying, would retain its mud cracks upon rewetting, either 

 by rain previous to flooding or by the flood waters themselves." 

 When the newly deposited layer is a very thin one it will curl up 

 like shavings on drying and these clay shavings will be blown into 

 the sand dunes, where, upon subsequent softening, they will be 

 compressed into clay lentils or pebbles, and so become a constituent 

 part of an otherwise pure sandstone. 



Preservation of Footprints, Etc., in Subaerial Deposits. 

 Of the greatest significance is the relative ease with which tracks 

 of animals are preserved in desert deposits. The scarcity of rain 

 permits their almost indefinite retention on suitable surfaces with- 

 out being buried. In the Sahara desert tracks of camels made in 

 1877 were still perfectly recognizable in 1892 (Foureau-21 :i/3), 

 the interval of fifteen years having altered them but little. Where- 

 ever (Walther-57 :6'6') a temporary accumulation of water after 

 a desert rain attracts the varied desert fauna, or allows animals 

 living on the border of the desert to make extended excursions into 

 the flooded regions, their footprints will be left upon the impres- 

 sionable surface of mud. remaining after such an inundation. These 



