THE OCEAN AND ITS WORK 233 



coasts, however, are somewhat irregular, because the shores are 

 usually composed of heterogeneous materials, and also because all 

 parts are not equally attacked by the waves. 



The rate at which coasts develop depends both upon the character 

 of the rocks of which they are composed and upon their exposure to 

 the waves. The rate is also affected by the amount of sediment 

 carried in by streams, since if the quantity is large, so much of the 

 energy of the waves and currents is expended in removing it that the 

 shore is but slightly attacked. An excellent example of possible 

 difference in the rate of erosion is to be found on the coast of New 

 England, where the rocky coast of Maine is still in youth, while the 

 coast of Cape Cod, composed of soft glacial material, has been at- 

 tacked more effectively and is well advanced toward maturity. 



Deposition in Seas and Lakes 



Source and Extent of Land-derived Sediments. — The sediments 

 carried to the ocean by the streams and the fragments broken from 

 the shores by the waves are soon deposited on the sea bottom and 

 for the most part do not reach a greater distance from the shore 

 than ten miles, although some are carried to the edge of the conti- 

 nental shelf. Some material for these deposits is also furnished by 

 glaciers and the winds. Opposite great rivers such as the Amazon 

 and Ganges, however, sedin nts are swept out by their currents and 

 deposited several hundred iles from their mouths, as is shown by 

 fine sediments derived from he sea bottom 200 to 800 miles from the 

 shore. There are two princ <al reasons for this comparatively narrow 

 belt of deposition. The most important, as already noted (p. 130), 

 is that all sediments sink shortly after reaching quiet water ; and the 

 other, that fine sediments settle more rapidly in salt water, very fine 

 silts settling in salt water in one fifteenth the time that they do in 

 fresh water. 



Stratification. — Deposits in any one place seldom accumulate to a 

 great thickness under exactly similar conditions and are consequently 

 in layers (Fig. 227) ; that is, they are stratified. Stratification is pro- 

 duced (1) usually by a change from time to time in the character or 

 composition of the sediment. For example, if the deposition of clay 

 is interrupted for a short time during which currents bring in sand, 

 the beds of clay will be separated by layers of sand. (2) If, after a 

 layer of sand has been laid down, deposition ceases for a time and the 



