202 GEOLOGY. 



the inclined fore-set beds shown in Fig. 189. The material in suspen- 

 sion is carried farther, settles more gradually, and constitutes the hot- 

 tom-set beds (c, Fig. 189). In time the bottom-set beds, originally 

 deposited some distance beyond the debouchure, may come to be over- 

 lain by the fore-set beds, deposited at a later time. While the fore- 

 set beds are being deposited on the steep slopes of the delta, and the 

 bottom-set beds beyond, deposition is also taking place on the top of 

 the delta. These top-set beds are laid down in a nearly horizontal posi- 

 tion, and their seaward margin is gradually extended. Thus the delta 

 comes to have the threefold structure sho^vn in Fig. 189. 



That part of the delta which is above the abrupt slope of its front 

 corresponds in all essentials to an alluvial fan ; but the delta as a whole 

 differs from the fan in its abrupt and crenate or digitate margin. 



It is to be noted that the delta is not Avholly the product of a stream's 

 activity. The stream supplies the material, but the lake or sea renders 

 at least passive assistance in its disposition. Not all rivers opening 

 into the sea build deltas, and their failure is often the result of waves 

 or shore currents which carry off the river sediment. Deltas are, how- 

 ever, sometimes formed in tidal seas, as at the debouchures of the Yukon ; 

 the Mackenzie, where the tidal range is three feet; the Niger, where 

 the range is four feet; the Hoang-Ho, where the range is eight feet; 

 and the Brahmaputra and Ganges, where the range is sixteen feet.* 

 Since lakes, bays, gulfs, and inland seas have weaker waves and cur- 

 rents than the open sea, they are more favorable than the latter for 

 the growth of deltas. Hence occur such deltas as those of the Mississippi, 

 the Nile, the Po, and the Danube. 



Deltas are likely to be absent, or confined to the heads of bays, on 

 coasts which have recently sunk. Their general absence on the Atlantic 

 coast of the United States is a case in point. 



The following figures give some idea of the extent of deltas, and of 

 their importance in land building. The Mississippi delta is advancing 

 into the Gulf at the rate of about 100 yards per year, or a mile in 16 or 

 17 years. Its length is more than 200 miles, its area more than 12,000 

 square miles, and its depth at New Orleans has been estimated at 700 ^ 

 to 1000 ^ feet. This great depth is believed to be the result of subsi- 



^ Davis. Physical Geography, p. 294. 



2 Humphreys and Abbot. Physics and Hydrauhcs of the Mississippi River. 



3 Corthell. Natl Geog. Mag., Vol. VIII, p. 351, 1897. 



