7IO PRINCIPLES OF STRATIGRAPHY 



around permanent lake bodies as the result of periodic shrinking 

 of the lake after a flooding of the adjacent lands. Such mud- 

 cracked areas will be exposed for long periods of time and so re- 

 semble the flood plains of rivers. They at best, however, form but 

 a narrow marginal belt around the lake and the beds characterized 

 by them would grade laterally into fresh water lake beds in which 

 remains of fresh water organisms are found. The mud cracks of 

 the Tertiary lake beds of Florissant may be of this type. 



River Flood Plains. Next in importance to the playa, and 

 perhaps even rivaling it in extent, is the river flood plain. Here 

 after a great flood extensive areas may be laid bare and be sub- 

 jected to desiccation and cracking during the long period of expo- 

 sure before the next flood. Since, in the lower reaches of rivers, 

 the material spread out by the flood is of the nature of a fine silt, 

 the conditions for the formation of the mud-cracks are fully satis- 

 fied. Here, too, preserval of the mud-crack record is readily ac- 

 complished by the filling of the fissures by the sediment of the next 

 flood. 



The mud-cracked flood plain deposit would dififer from the playa 

 deposit in the more frequent presence of carbonaceous material, since 

 vegetation is an accompaniment of river courses, but, as a rule, 

 absent from the playa, or found only around the margin. Aquatic 

 animals, too, should be more characteristic of the flood-plain than 

 of the desert deposit, and would especially characterize the old 

 stream channels dissecting the flood plain, and recognizable in the 

 fossil state by the lines of coarser sediment — the filling of these 

 channels — which traverse the finer deposits. 



The Shore Zone. The shore zone between high and low water 

 also may furnish conditions favorable for the formation of mud- 

 cracks. This is especially the case where, as in the Bay of Fundy, 

 the tide recedes very far, and where large tracts remain exposed 

 during the fortnightly interval between high spring tides. Along 

 the margins of estuaries broad mud-flats are often exposed, and 

 here shrinkage cracks may form between tides. The time of expo- 

 sure of all but the highest parts of the shore zone, is, however, too 

 short to allow of a sufficient hardening of the mud-cracked area 

 to enable it to resist the softening and destroying effects of the 

 returning tide. Moreover, in modern mud flats of this type, or- 

 ganisms exist in great numbers, so that we would expect to find 

 mud-cracked rocks which are formed on the shore to be more or 

 less fossiliferous. 



On the whole, mud-cracks are much more characteristic of con- 

 tinental deposits, especially of the playa and flood plain, than of 



