THEORY OF FORMATION OF DELTA CUSPS. 



419 



side current lines in figure 12. A rounded delta may thus become a cus- 

 pate one. If the ratio between river and sea increases in favor of along- 

 shore action a cuspate delta may be changed 

 to a rounded one. 



Included marsh areas are drawn in this 

 type because where a dominant movement 

 alongshore is indicated on the maps such 

 areas usually occur. They do not neces- 

 sarily result from this hypothesis of growth, 

 and may occur in any delta where the ad- 

 vance ha.s been by leaps. 



The third method of formation is where 

 two currents move alongshore, one from the 

 right and the other from the left, toward 

 the mouth of the stream. The river may 

 act in this case as the projection of the land 

 at the beginning of a new cycle was sup- 

 posed to do in starting 

 the growth of a current 

 cuspate foreland (page 

 403). 



The three methods Growth 



of growth can doubt- Current. 



less be easily distinguished upon the ground. It 

 is not so easy upon the maps, for the main indica- 

 tion as to current upon the delta foreland is the 

 deflection of small streams. Such streams often 

 do not exist. Offsets and overlaps are not com- 

 mon upon delta forelands. If there is a dominant 

 current along a coast a delta occurring upon it may 

 with great probability be referred to the second 

 method of growth. Like so many other things in 

 this world, this is wholly a question of ratios, and 

 in any given delta there will be some on and off 

 shore action and some alongshore action. 



Figure w.— Ideal Stages of Delta 

 with dominant alongshore 



Figure 12. — Ideal Stages 

 of Delta Growth with two 

 alongshore Currents toward 

 Delta. 



TYPE. 



The typical example of a cuspate delta is given 

 in figure 13. The two gently swinging shore curves concave seaward with 

 their dune-lined beaches are the work of the sea. At the point of intersec- 

 tion of these two curves the river mouths. The form of the land shows that 



