ACCUMULATION OF SEDIMENTS 215 



more than a few score miles from the point where it enters 

 any large water basin. In the greater streams the quantity of 

 this detritus which is carried to the sea is often very great. 

 Thus the Mississippi sends out each year somewhere near one- 

 twentieth of a cubic mile of undissolved material, the whole of 

 which doubtless comes to rest on the broad margin of the 

 ofrowinof delta ; and the Amazon, owinof to the ereater rainfall 

 of its basin, probably contributes several times as much sedi- 

 ment to the sea-fioors near its mouth. The result of this 

 effusion of sediment into the basin near the mouth is that all 

 rivers which pour forth turbid waters are apt to have wide 

 shallows without very distinct channels at their points of exit. 

 Where, as in the case of the Amazon, the entrance to the delta 

 mouth is not thus obstructed, a condition which rarely occurs, 

 we have to account for the fact either by a recent subsidence 

 of the land or through the action of powerful currents such as 

 are produced by tides, cooperating", it may be, with oceanic 

 movements brought about in other ways. 



Wherever a stream conveying a considerable amount of 

 detritus enters any bay or other indentation of the coast, we 

 find indication of its damaging action upon the harborage. 

 Beginning at the head of the reentrant it constructs a delta- 

 like accumulation, which gradually converts the basin into 

 marshy land. In front of the distinct delta there is a wide, 

 gently sloping surface over which the finer mud is laid down. 

 If a part of the debris escapes from beyond the harbor mouth 

 the currents and waves of the shore, acting in a manner here- 

 after to be described, are apt to build the accumulation into the 

 forni of bars or spits which may more or less close the entrance 

 to the port. In this way some of our more important havens, 

 such as that of Mobile Bay, are subjected to a constant 



