372 



GEOLOGY, 



Again, if the coast be sinking, new deposits of coarse material may be 

 made on older ones. In this way also great thicknesses of sediment 



Fig. 326.— Rill-marks. Same locality as 325. (Walcott.) 



may be accumulated, all parts of which were deposited in shallow water. 

 The great thickness of some of the conglomerate beds of the past shows 

 how far this process may go. 



As a rule, no definite line marks the seaward terminus of the coarse 

 detritus, since coarse material is carried farther out when the waves 

 run high (and the undertow is strong) than when they are feeble. In 

 calm weather, therefore, fine sediment may be deposited where coarse 

 had been laid down in the preceding storm, only to be covered in turn 

 by other deposits of a different character. Thus gravel grades off into 

 sand, with more or less overlapping or interwedging, and sand grades 

 off into silt in the same way. This is diagrammatically illustrated by 

 Fig. 323. 



