CLIMATE AND TERRESTRIAL DEPOSITS 279 



sional rains or floods. The diagrams given by Hilgard show that in 

 clay lands the bulk of the alkali is within two feet of the surface and 

 in sandy lands within seven feet.^ Mcdlicott and Blanford state 

 that in the worst alkali tracts of India sweet water is obtainable at 

 depths below sixty to eighty fect.^ Except in the most arid regions 

 the soluble alkali salts are thus seen to be prevented from accumulating 

 through the strata. 



Combinations of Fluvial and Aeolian Structures. — The surfaces of 

 arid flood-plains and to a lesser extent those of semiarid climates are 

 dry and barren for a considerable portion of each year, and the detri- 

 tus becomes reworked by wind action, with the result that in flood 

 plain deposits of desert climates fluvial, pluvial, and aeolian forma- 

 tions are all of wide occurrence and brought into immediate juxta- 

 position, producing combinations of structures which may be more 

 or less readily recognized in the buried strata. The clayey layers 

 deposited by flood waters crack upon drying into polygonal discs 

 which curl upward on their edges, giving concave mud-cracked 

 surfaces. The fine silt and sand, not possessing coherency when dry, 

 and not being held by vegetation, give rise to intolerable dust storms. 

 The finer silt may cover the adjacent regions with loess and the 

 coarser portions, derived from the stream channels, may be the source 

 of widespread dune sands, such as mantle the deltas of the Indus and 

 the Helmund, burying many ancient cities and subjecting the still 

 exposed ruins to aeolian scour. 



The more distinctive structures resulting from the combinations 

 of water and wind action may be classified and described as follows: 



First, mud-cracks -filled with aeolian sands. — Silt and sand will be 

 blown over and fill up the cracks developed by the drying of argilla- 

 ceous water-laid deposits. Consequently, the sand is filled in under 

 the raised rims of the polygonal discs and becomes continuous with 

 the mantle of sand above. In this way, the concavity upward of 

 the individual plates is preserved and the mud-cracks are not obhter- 

 .ated, even in a silty clay which would slack and crumble immediately 

 upon being re wet by the advancing waters of the following inundation. 

 Experiments by the writer go to show that the upturned edges of the 



' Soils, 1906, chap. xxii. 



3 Geology of India, Part I, 1879, p. 413. 



