590 



PRINCIPLES OF STRATIGRAPHY 



largely composed of fine sediments; the proportion of fine to coarse 

 materials in the extensive deposits of these rivers seems to be 

 greater than it is in many of the so-called lake beds of the west 

 [of North America]." {Davis-ig -.^61-362.) 



River flood plains consisting of fine material are especially 

 adapted for receiving the transit impression of v organisms as well 

 as sun-cracks, rain-drop impressions, etc. Indeed, almost the only 

 known modern examples of such structures commonly referred to 

 seashore origin are found on river flood plains or on the surfaces 

 of the playas. 



Another feature characterizing many river flood plains is the 

 levelness of their surfaces, which argues well for the close approxi- 

 mation to horizontality of their strata. Many minor irregularities 



Fig. 123. Torrential type of cross-bedding ; seen in modern torrential de- 

 posits. (After Hobbs.) 



will, of course, be found, such as shallow channels, filled by later 

 deposits and other evidence of contemporaneous erosion. 



Cross-Bedding of Torrential Sediments. Whenever sands and 

 gravels are spread by torrential floods, diagonal or cross-bedding on 

 a large scale will result. Between the nearly horizontal layers of 

 finer sands or gravels occur beds inclined at a nearly uniform and 

 relatively large angle with the enclosing layers. Successive cross- 

 bedded strata will have their beds slope in the same direction, 

 which is that of the stream producing them. Such a type of cross- 

 bedding is wholly inconsistent with the theory of wave formation 

 or of currents in a body of water. It has been observed in modern 

 torrential deposits (Hobbs-31 :2()i), and is a characteristic feature 

 of many ancient sandstones and conglomerates, the torrential origin 

 of which it indicates (Fig. 123). It is not an infrequent structure 

 in the torrential deposits of the terminal moraine on Long Island. 

 (See further, Chapter XVII. > 



Thickness and Composition of River Deposits. The thickness 



