GEOLOGICAL IMPORTANCE OF SEDIMENTATION 533 



and subject to annual overflow. Both are characterized by sun- 

 cracks — a characteristic not ascribed to the other types of soil. It 

 is noteworthy that the purer clay, and therefore that more favorable 

 for the preservation of mud-cracks, is deposited, not from flowing, 

 but rather from stagnant back waters, the one at high-water stage,- 

 the other when the water-level is lower. Only a portion of the 

 deposits of the larger flood-plains are, therefore, well fitted to retain 

 surface impressions until buried, and this principle must be carried 

 into the past. 



It is seen that mud-cracks may originate in pluvial climates, but 

 the thick mat of vegetation apt to form under such conditions would 

 tend, wherever it existed by the binding action of the roots, to pre- 

 vent cracks from forming. An arid valley climate, therefore, and 

 abundant sediment would be more favorable conditions for the 

 broad development of mud-cracks. Rock decomposition, rather 

 than disintegration at the sources of supply, the mark of a pluvial 

 climate, should, on the other hand, be favorable as furnishing a 

 larger proportion of pure clay mixed with the coarser material. 



In conclusion, it is seen that special conditions are necessary for 

 a complete temporary preserval of mud-cracked surfaces even where 

 continued sedimentation without intervening erosion occurs. In 

 formations which show traces of mud-cracks it is to be anticipated 

 that other, more or less argillaceous layers may also have been 

 exposed to the air. Sediments swept along by broad, slow-moving 

 waters will ordinarily possess too much loam for the good preserval 

 of mud-cracks. But where the flood waters stand quietly before 

 being drained away, or where the loam is strained out or settles at 

 another place, the fine clay will settle, forming a deposit free from 

 sand, and capable of retaining even the faintest markings made 

 upon its drying sufrace. 



MUD- CRACKS OF PLAYAS 



Description. — The characteristics of these may be best appre- 

 ciated by quoting from Russell's descriptions of the present and 

 extinct lakes of Nevada.^ Speaking of the ephemeral lakes forming 

 either after the rainy season or even after a single storm he says : 



I The Physiography of the United States (American Book Co.), Monograph 4, 

 pp. 105-10. 



