630 



GUILCHER 



[chap. 24 



of sand or silt are (loi)ositftl ujtoii tlic clays, (uillyiiig and coarse lenses in 

 clay beds are thus common. The counterpart of these deposits in the emerged 

 part of the delta is a subaqueous offshore deposition, including topset beds 

 which form a shallow submarine ])lain, foreset beds extending outwards and 

 di])])ing downwards in inclined and parallel strata, and bottomset beds on the 

 sea-floor beyond the submarine delta front (Fig. S). As a rule, the foresets are 

 coarser than the topsets, and the bottomsets are the finest ones; and, as the 

 delta is prograded, the foresets gradually overlap the older bottomsets. Actually, 

 this is an idealized picture which may be found in small deltas but rather 

 infrequently in large ones, where it is disturbed in various ways. On the east 

 margin of the Mississippi delta (Fig. 17), which has been thoroughly investigated 

 by Shepard (1956), the foreset beds, or "pro-delta slope", are definitely finer 

 than the bottomset and topset beds, with a percentage of clay as high as 55%, 

 and their dip scarcely exceeds 1 degree, whereas it reaches commonly 10 to 25 

 degrees in smaller deltas. In the submarine part of the Rhone delta (Van 



Fig. 8. Schematic development of the subaqueous parts of a small delta. 



Straaten, in Russell, 1959), the recent foreset beds are much steeper and coarser. 

 In both areas, submarine gullies in the delta front complicate the picture. 



A major phenomenon in delta evolution is the shifting course of the river into 

 successive distributaries, which may lead to the building of successive sub-deltas. 

 As the delta protrudes more and more into the sea, the average slope and the 

 capacity decrease, while much shorter routes can be found across the swamps. 

 An occasional crevasse initiates a new course, and a new sub-delta comes into 

 existence, till it in turn is abandoned, and so on. But the inactive parts are 

 quickly eroded by sea weaves, since they are no longer fed with sediment by the 

 river, and the balance between sea and river forces then favours erosion. The 

 Mississippi delta is classic in this respect: within the last 5000 years the river 

 has built five large sub-deltas (Fig. 9). The oldest ones (Cypremort and Teche) are 

 the most decayed; the intermediate ones (Lafourche and Saint Bernard) have 

 subsided below sea-level in their outer parts and barrier islands have been 

 built by waves in front of them with the sand fraction. These islands are rapidly 

 migrating landwards, but the old river courses (the so-called bayous) and levees 

 remain as yet conspicuous. Even the ])resent sub-delta (Balize) is not fully 

 active in all its ])arts, and some of its distributaries tend to become tidal creeks 

 (Welder, 1959). Other good exam])les of shifting courses are found in the Med- 

 jerda delta, Tunisia (Pimienta, 1959), which is also undergoing subsidence, and 



