420 



F. P. GULLIVER — CUSPATE FORELANDS. 



if it were not for the river there would not be any cusp here, as there is no 

 projecting point in the okUancI to cause eddies in the currents. The evi- 

 dence from the turning of the mouths of the small streams both to right 



and left indicates that the direc- 

 tion of current motion alongshore 

 is probabl}'' sometimes in one di- 

 rection and sometimes in the re- 

 verse. The smaller streams on 

 each side of the river mouth are 

 deflected away from the point of 

 the cusp, indicating that the delta 

 mass divided an on-shore current 

 and turned it to the right and left, 

 carrying the river sediment from 

 the river along the shore. Further 

 from the river, both on tlie right 

 and left sides, there are streams 

 deflected toward tlie mouth of the 

 main stream. There is here evi- 

 dently no dominant movement in 

 either direction alongshore. 



A former stage of the delta is 

 indicated by the ridge of geolog- 

 ically older material, which is 

 represented in the figure by the 

 broken line. This earlier stage of 

 the delta front is seen to liave a 

 rounded outline. This suggests 

 that formerly tliere was a domi- 

 nant movement alongshore. Back 

 of this former shon.'line are seen 

 f .... ,^ ... ■;^ '^ ares of marsh, filled lagoons, or 



v\oxiv.^ n.-Typc delta ciispaic i-urciand. lowhiud bchiud the old bcach. 



Since this leap from some still earlier i)Osition of the shoreline, the for- 

 ward growth seems to have been gradual, for no long slashes of swamp 

 are shown. From map inspection this delta cannot be definitely referred 

 to either of these hypotheses of origin, for it combines features of both 

 the first and the second. 



This type is the Tiber (Ital., 149; Carta Geologica della Campagna 

 Romana, Roma, 1888). It has been turned from the usually desirable 

 north and south orientation parallel to the sides of the page in order to 



Y^Llonxetre^ 



