THE WORK OF THE OCEAN. 



373 



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 hkely to 

 be ripple- and rill-marked (Figs. 324, 325, 326), and cross-bedding 



Fig. 327.— Cross-bedding. (Gilbert.) 



(Fig. 327) is of common occurrence. Clayey sediments accumulated 

 between high and low water are often sun-cracked (Fig. 328), and the 

 tracks of land animals are sometimes preserved on their surfaces. Shal- 

 low-water deposits often contain fossils of organisms which live in waters 

 of sHght depth. These characteristics are sufficient to differentiate 



Fig. 328. — Sun-cracks. These cracks were on the mud-flats of the Missouri a few 

 miles above Kansas City, but the sun-cracks on shore-deposits are not essentially 

 different. (Calvin.) 



sedimentary formations made in shallow water from those made in 

 deep water, even after they have been converted into solid rock and 

 after the rock has emerged from the sea. Many of these characteristics 

 are, however, shared by deposits made by streams on the land. Sub- 

 aerial and lacustrine sediments are usually distinguishable from those 

 made in the sea by their fossils, and sometimes by their distribution. 



