256 PHYSICAL GEOLOGY 



Distribution. Fine dust from the land is carried by winds 

 to all parts of the ocean. The deposition in the ocean of 

 land-derived sediments is accordingly as widespread as the 

 sea itself. This has undoubtedly been true ever since the 

 oceans were formed. But most of the waste from the land 

 settles within two or three hundred miles of the shore. Cer- 

 tain powerful river currents carry material much farther out 

 to sea. The Kongo River, for example, is said to project its 

 current 600 miles from shore, and the Ganges River nearly 

 1000 miles. Such cases are, however, very exceptional. 

 The marginal sea bottoms are the great areas of sedimentation. 

 The distribution of the important land-derived (terrigenous) 

 deposits is shown by Figure 276. 



Continental shelves. Parts of the continental shelves 

 (p. 233) may be to some considerable degree a product of the 

 long-continued offshore accumulation of the waste of the land. 

 Obviously, however, a continental shelf might be formed by 

 the submergence of a coastal plain, due either to its depression 

 or to an elevation of the surface of the sea. Soundings along 

 the eastern coast of the United States and elsewhere have 

 shown that valley depressions extend across the continental 

 shelf from the mouths of various rivers. Since these valleys 

 must have been cut above sea level, the sections of the con- 

 tinental shelf in which they occur appear to be due to the 

 submergence of former land areas. Continental shelves may 

 also be in part plains of marine denudation. 



Character of land-derived deposits. The more important 

 points concerning both the character and the structure of de- 

 posits of land-derived sediments were noted in the discussion 

 of sedimentary rocks (pp. 35-37). It will be remembered that 

 the gravel, sand, and mud are shifted about and worked over 

 by waves and currents, often for long periods before reaching 

 a final resting place. In the process the materials are sorted, 

 and beds of each kind result, which grade into one another 

 both vertically and horizontally. Most of the gravel comes 

 to rest close to shore in depths of 50 feet or less, but fine sand, 



