186 PHYSICAL GEOLOGY 



quiet sea water. Thus levees develop, and the delta is 

 built above the level of the sea. As the original channels 

 across the delta gradually fill with sediment, some of the 

 water breaks over the sides, following new courses to the sea 

 and building up the different parts of the delta in turn. 

 By this means a complicated system of distributaries may 

 be formed (Fig. 195). Portions of the shallow sea covering 

 the submarine platform are sometimes inclosed by the river 

 deposits or between them and the old shore line, forming 

 delta lakes. This was the origin of Lakes Borgne and 

 Pontchartrain, on the delta of the Mississippi near New 

 Orleans (Fig. 193). Extensive deposits of peat may accu- 

 mulate in delta lakes and swamps. Apart from the shallow 

 basins of their lakes and marshes, and the low ridges along 



their distributaries, the 

 land surfaces of great 

 deltas are nearly level, 



FIG. 196. - Profile and section of a delta. continuing the glope Q f the 



flood plain farther up river. The upper beds of a delta, de- 

 posited by the river upon the submarine flat, are nearly hori- 

 zontal. Deltas are accordingly characterized by three sets 

 of beds (Fig. 196). The bottom and top beds are nearly 

 horizontal, while the middle beds are inclined more or less 

 steeply. 



Deltas grow at very unequal rates. The ratio between the 

 volume of sediment brought by the river and the strength of 

 waves and currents off the river mouth is a chief determinant. 

 The Mississippi brings down about 7,500,000,000 cubic feet 

 of sediment a year; and as the tides of the Gulf of Mexico 

 are weak, the delta is being extended seaward off the mouths 

 of the main distributaries at the rate of about a mile in sixteen 

 years. It appears to have grown at about this rate for many 

 years. An English writer reported in 1770 that the Balize, a 

 small fort built by the French on a little island which was at 

 the mouth of the river in 1734, was then two miles up. The 

 depth of the water into which a delta is being built also in- 



