THE WORK OF STREAMS 185 



(Plate II); Dubuque, Iowa; and Hartford, Connecticut, are 

 examples. Miller and Crown City (Plate IV) are examples 

 of hundreds of villages situated similarly. 



Deltas. Some of the material which rivers bring to the 

 sea or to lakes is carried away by waves and currents ; much 

 of it often accumulates off the mouths of the rivers, especially 

 if they flow into tideless or nearly tideless bodies of water. 



FIG. 195. The delta of the Alsek River, Alaska. Shows numerous dis- 

 tributaries. (Netland, U.S. Boundary Commission.) 



Such deposits may form deltas (Fig. 93). Deltas are so 

 called from the Greek letter (A) of that name, whose shape 

 they occasionally resemble (Fig. 194). 



As the current is checked at the mouth of a river flowing 

 into the sea, the coarsest of the sediment is dropped first, 

 forming slanting beds, whose angle of slope is determined 

 largely by the size and shape of the material. The finer 

 sediment settles less rapidly, and is spread by waves and 

 tides over a larger area in nearly horizontal sheets. As dep- 

 osition continues, the steeper beds of coarser material are 

 built out upon the nearly level beds of fine. When this 

 submarine embankment is built up close to the surface of 

 the water, it becomes in effect an extension of the river bed, 

 across which the projected current rolls and drags material, 

 and upon which it deposits a part of its load. Deposition 

 on the submarine platform is most active along the edges 

 of the river current, because of friction with the relatively 



B. & B. GEOL. 11 



