GEOLOGICAL IMPORTANCE OF SEDIMENTATION 341 



possessing bathymetric contours shows that the Amazon, facing the 

 open ocean, and subject both to wave-erosion and tidal scour, has a 

 submerged delta which for 200 kilometers is less than 20 meters 

 beneath the surface of the sea, while the aerial portions belong rather 

 to the flood-plain, since they do not extend eastward beyond the 

 adjacent margins of the continent. On the contrary, in the case of 

 the Nile, the Danube, the Po, and the Mississippi, flowing into rela- 

 tively quiet and tideless seas, practically all the foreset beds are above 

 water, and thus the greater part of the area of the river delta, neglect- 

 ing the attenuated bottomset beds, is a region of subaerial — that is, 

 of continental — sedimentation. 

 According to Forshey 



more than two-thirds of the Mississippi delta in the ordinary state of the river 



are above water But if the river were unrestrained by levees, the highest 



floods would fill the alluvial basin and make a sea 600 miles long, 60 miles in 

 mean width, and 122 feet in mean depth.' 



As another illustration, 



the delta of the Hoang Ho (Yellow River) extends along the coast from near 

 Peking, on the north beyond the Pei Ho, to Hung-tse Lake on the south, where 

 it joins the plains of the Yang-tse-Kiang. The distance is 400 miles, but the 

 mountainous province of Shan-Tung is to be excluded. From the coast the delta 

 extends westward for 300 miles. The river is here useless for navigation. The 

 whole delta region would be under water during flood seasons except for drainage 

 by artificial canals and dikes of great length.^ 



Under natural conditions every flood would, by the settling of 

 mud or sand from the broad flood-waters, contribute to the upbuild- 

 ing of the delta plain; as, for example, the statute of Rameses II at 

 Memphis has been buried in about 9 feet of river deposit in somewhat 

 over 3,000 years. The interference of man has, however, doubtless 

 here changed the natural rate. 



The geological importance of this aggradational work of rivers 

 over their deltas is obvious. During periods of rapid subaerial 

 denudation an appreciable volume of the sediments removed from 

 the interior of the land should be laid down, not beneath the sea, but 

 as a land surface facing and encroaching upon the salt waters. Such 

 a character of river deposition would be at a maximum in a shallow 



1 J D. Dana, Mamml of Geology, p. 197; quoted from C. G. Forshey, 1873. 



2 Ihid., p. 198. 



