THE WORK OF THE OCEAN 



319 



shallow water may be extended seaward by the aggradation of the 

 bottom, shallow-water deposits may cover extensive areas. They 

 may become deep at the same time, for as the outer border of the 



Fig. 262. Diagrams showing how shallow-water deposits may attain con 

 siderable depth by the shifting of the zone of deposition seaward. 



shallow-water zone is shifted seaward by aggradation, the vertical 

 space to be filled becomes greater (compare A and B, Fig. 262). 

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

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

 sediment may be accumulated, all parts of which were deposited 

 in shallow water. The great thicknesses of some of the conglom- 

 erate beds of the past show how far this process may go. 



Fig. 263. Ripple-marks. 



