228 



PROCEEDINGS OP THE AMERICAN ACADEMY. 



Holland (Atlas Univ., 26) is in large part a great confluent delta formed largely 



by the Rhine and adjacent streams. 



The Nile is a raulti-cuspate-lobate delta (Brit. Ad. 2673, 2630; Credner, loc. 



cit., 2). 



3. Cuspate Deltas* — The typical example of a cuspate delta is given in Figure 29. 



The two gently swinging shore curves, concave seawards, with their dune-lined 



beaches, are the work of the sea. 

 At the point of intersection of these 

 two curves the river empties. The 

 form of the land siiows that, if it 

 were not for the river, there would 

 not be any cusp here, as there is 

 no projecting point in the oldland 

 to cause eddies in the currents. 

 The evidence from tiie turning of 

 the mouths of the small streams 

 both to right and left indicates 

 that the direction of current motion 

 alongshore is probably sometimes 

 in one direction, and sometimes in 

 the reverse. The smaller streams 

 on each side of the river's mouth 

 are deflected away from the point 

 of the cusp, indicating that the 

 delta mass divided an onshore 

 current and turned it to the right 

 and left, carrying the river sediment 

 from the river along the shore. 

 Farther from the river, both on 

 the right and left sides, there are 

 streams deflected toward the mouth 

 of the main stream. There is here 

 evidently no dominant movement 

 in either direction along the shore. 



A former stage of the delta is 

 indicated by the ridge of geologi- 

 cally older material, which is repre- 

 sented in the figure by the broken 

 line. This earlier stage of the delta 

 front is seen to have a rounded 

 — outline. This suggests that for- 



Tiber, merly there was a dominant move- 

 ment alongshore. Back of this 

 former shoreline are seen areas of 



marsh, filled lagoons, or lowland behind the old beach. Since this leap from 



some still earlier position of the shoreline, the forward growth seems to have been 



Kilometers 



Figure 29. 



Typical Cuspate Delta 

 Italy. 



* For fuller discussion, see Bull. G. S. A., 1896, VII. 417-421. 



