320 



GEOLOGY 



Characteristics of shallow-water deposits. Clastic sediments 

 laid down in shallow water have several distinctive characteristics. 

 While they are, in the aggregate, coarse, they are characterized 

 by frequent variations in coarseness. The surfaces of successive 

 beds are likely to be ripple- and rill-marked (Figs. 263 and 264), 



Fig. 264. Rill-marks resembling impressions of seaweeds. Beach at Noyes 

 Point, R. I. (Walcott, IT. S. Gcol. Surv.) 



and cross-bedding (Figs. 265 and 266) is common. Clayey sedi- 

 ments accumulated between high and low water are often sun- 

 cracked (Figs. 267 and 268), and the tracks of land animals arc 

 sometimes preserved on their surfaces. Shallow-water deposits 

 often contain fossils of organisms which live in waters of slight 

 depth. These characteristics are sufficient to differentiate sedi- 



