THE WORK OF STREAMS 131 



delta has been built upward and outward in spite of subsidence; 

 the sinking which produced the Chesapeake and Delaware bays, 

 however, was so rapid that estuaries were formed (p. 114). The Mis- 

 sissippi River is extending its delta at the remarkable rate of one 

 mile in sixteen years ; the Rhone has added a mile to its delta in 

 Lake Geneva since Roman times. It leaves the lake as a clear 

 stream, but gathers sediment from its tributaries in France, with 

 which it builds another delta at its mouth at the rate of about a mile 

 a century. In 220 B.C. the town of Pu-tai, China, stood one third of a 

 mile from the sea, but in 1730 it was 47 miles inland, and to-day it 

 is 48 miles from the shore. (King.) Many of the " points " in lakes 

 are deltas which have been built out by streams. 



Structure of Deltas. — A section through a delta shows approxi- 

 mately horizontal beds of fine material at the bottom, which do not 

 differ greatly from other 

 deposits at a similar 

 depth where no delta 

 occurs. These are termed 

 the bottom-set beds. 

 Above these are the Fig. I2I# — Ideal section of a delta built into 

 , ■ i- j /■ , quiet waters of constant level. The lower horizontal 



steeply inclined fore-set beds are callfed bottom . set> the inclined) fore . set) and 



beds, composed of coarser the upper, top-set. (After Barrell.) 



sediments which have 



been swept outward by the currents and waves and may have a 



slope approaching the angle of repose (Figs. 121, 122). The top-set 



beds are nearly horizontal and are laid down upon the fore-set beds. 



These are usually the last deposits of the river in the upbuilding of 



the delta. 



The surface of a delta is comparatively level, but gradually rises 

 upstream. In it large and small lakes may occur; the depressions 

 in which they lie being those portions of the delta which, because of 

 the accidental position of the distributaries, were not filled to the 

 general level with sediment. Their life is necessarily short, since 

 they are gradually being filled by accumulations of silt during floods, 

 and by swamp vegetation. 



Deltas may be very extensive. That of the Ganges and Brahma- 

 putra has an area of 50,000 to 60,000 square miles, with its head 

 200 miles from the sea. The length of the Mississippi delta is more 

 than 200 miles, and its area is more than 120,000 square miles. 

 The Orinoco delta has an area larger than that of New Jersey. The 



