340 STUDIES FOR STUDENTS 



this conclusion is not obtained so much from the nature of present 

 movements as from the evidence of the upturned and eroded strata 

 themselves, deposited as the geosynclinal axes sank. 



Where there is no subsidence, as is to be expected in the case of 

 the smaller deltas, the subaerial character is readily maintained and 

 extended as a result of delta-building, but the subaerial deposits 

 cannot extend below the water level. Where subsidence accompanies 

 sedimentation, however, the subaerial character of the delta tends to 

 be destroyed ; but if the river is able to build its plain upward as fast 

 as the downward movement takes place, the subaerial deposits may 

 reach to any depth. In the larger deltas which have been tapped by 

 boreholes this appears to be the case. 



FACTORS GOVERNING DELTA LAND SURFACES 



The strata of a delta have been classified into the bottom-set, fore- 

 set, and topset beds;^ the first consisting of fine material deposited 

 from suspension on the bottom of the sea beyond the main portion of 

 the delta, the foreset beds comprising the steeply inclined portion con- 

 sisting of slightly coarser detritus, and the flat topset beds being the 

 result of the aggradation work of the river, building up its stream to 

 grade as the front of the delta advances farther outward, or as the 

 whole slowly subsides. The tcpset beds are largely of subaerial ori- 

 gin, though the delta is fronted for a short distance by a shallow sub- 

 merged platform, across which the detritus is carried to deeper water. 

 Of these three portions, the foreset beds usually comprise the greater 

 volume of the deposits, but the thinner bottomset and topset spread 

 over the greater area. 



The ratio of the submerged to the emerged portions of the topset 

 beds depends upon a number of factors. A rapid subsidence may 

 carry the whole beneath the sea, but where subsidence is slower than 

 upbuilding, the proportion of the topset surface which is submerged 

 will depend upon the balance of power between the waves, tides, and 

 currents on the one hand, and the constructive work of the river on 

 the other. 



The depth of water over the submerged portion of the delta also 

 depends upon the strength of the waves. Thus, an inspection of maps 



I . Li Chamberlin and Sabsbury, Geology, Vol. I, p. 191. ' 



