564 PRINCIPLES OF STRATIGRAPHY 



Origin of Intercalated Dust and Clay Layers, and of Clay Balls in 



Sand D lines. 



The depressions between the sand dunes when not occupied by 

 standing water, as in the moist sand-hill region of Nebraska, often 

 represent a flat playa or takyr surface, the dried mud bed of a 

 temporary lake, which evaporated soon after its formation under 

 the influence of a dry climate. Such surfaces are common in most 

 desert regions, and they are not infrequently characterized by mud 

 cracks, footprints and trails of various animals. When the sand 

 dunes advance over such a surface a horizontal bed of clay will 

 separate the diagonally bedded eolian sands. This clay bed will 

 of course be of limited extent only, wedging out around the margin 

 of the playa. If the clay layer of the playa surface is very thin 

 (i to 2 cm.), the pieces into which it splits on drying will curl up 

 like shavings, and in such a condition they may be blown by the 

 wind into the dune. Here, on becoming moistened by the winter 

 rains, they will be compressed into flat lenticles of clay and form 

 the "clay galls" so common in modern as well as ancient eolian 

 deposits. 



Clay and dust accumulations are not confined, however, to the 

 depressions between the dunes. A light wind, active for a period of 

 time, may dust over the dune with a coating of argillaceous par- 

 ticles, which then become incori)orated in the sand mass as oblique 

 partition layers. Such occurrences are not uncommon in the Trans- 

 caspian (Kara Kum) and other deserts. They appear as steeply 

 inclined sheets of clay penetrating the dunes, and are not infre- 

 quently found in the form of parallel "clay dikes," 10 to 15 meters 

 in length, in the interdune areas, where they may rise a centimeter 

 above the flat surface, representing the residual base of a cross- 

 bedded dune, the top of which has moved onward. 



Peat and Lignite Deposits in Sand Dunes. 



Shore dunes often transgress across peat deposits formed in 

 swamps or marshes behind the dunes. Such peat deposits, some- 

 times with old tree stumps, are found on the shores of Nantucket, 

 and are sometimes buried under deep masses of dune sand, as at 

 the Nauset lights on Cape Cod. They are characteristic of other 

 regions as well. Forests buried by advancing dunes are killed, 

 and the wood, if buried long enough, is converted into lignite. The 



