THE WORK OF RUNNING WATER. 



201 



until some such system of currents as shown in Fig. 190 is developed. 

 The result is that a delta's growth is not simply in the line of extension 

 of the main stream, but in a more or less semicircular area, the center 

 of the circle being a point slightly below the position of the debouchure 

 of the stream when the delta began. At any stage in its development 

 the margin of the delta is more or less crenate (Fig. 191), or character- 

 ized by delta fingers (Fig. 190), the projections corresponding to the 

 positions of the debouchures of the latest streams flowing across it. 

 The extreme ends of the delta lobes (Fig. 190), and of groups of the 

 delta fingers, often have something of the shape of the Greek letter from 

 which the name originated, but the resemblance in form between a 

 well-developed delta and the Greek letter is not striking. Deltas are 



Fig. 191. — A miniature delta. 



sometimes built in bays, and in such cases their forms are predeter- 

 mined on all sides but one. The head of a delta is sometimes arbitrarih^ 

 located at the point where the first distributaries are giA^en off. Since 

 this point shifts widely with time, the definition can hardh^ be accepted. 

 On this basis the head of the Mississippi delta is about 200 miles above 

 its lower end. In reality it is much farther north. 



The structure of a delta, showTii in Fig. 189, shows its history. At 

 any stage in its growth the river discharges its sediment across that 

 part of the platform already built. The sediment rolled at the bottom 

 of the current is dumped on reaching the steep slope, and constitutes 



