344 STUDIES FOR STUDENTS 



water, and exposed to heavy tides and storms. For example, Tremen- 

 heere has shown that detritus from the Indus is swept northwestward 

 along the coast beyond the limits of the delta by the winds of the 

 southwest monsoon/ The monsoon winds of the Arabian Sea are 

 reversed during the year, with a resulting reversal of ocean currents; 

 and this exposure of the delta front to the alternating and powerful 

 waves, tides, and currents is doubtless the cause of the fact, made 

 known by Murray, that terrigenous deposits extend 800 miles from the 

 mouth of of the Indus and cover an area of more than 700,000 square 

 miles. ^ 



Under such circumstances the materials which in a quiet sea 

 would contribute to make the foreset beds and advance the delta 

 must be largely swept along shore and out to sea, contributing to 

 widespread bottomset beds. 



Excluding these widespread portions from the delta proper, it is 

 seen that under these conditions the topset beds of subaerial origin, 

 while not contributing a larger volume than before, may form a 

 larger ratio of the entire delta deposit. On the other hand, as the 

 delta advances into deeper and open water, a larger amount of material 

 is deposited beneath the surface on the delta front, in order to build 

 it out; and this also would cause the delta to advance more slowly, 

 but diminish the ratio of the land-formed deposits. When a delta 

 has reached a stationary limit, it is only possible for superficial deposits 

 to be deposited through subsidence; but as this appears to be not 

 unusual to such areas at the present time, it is also to be expected in 

 the past. 



The diagrams, furthermore, indicate that the ratio of subaerial to 

 marine deposits in an advancing, but not subsiding, delta depends for 

 one factor upon the relative gradient of the topset to the foreset beds. 

 The gradient depends upon a number of factors which cannot be 

 fully discussed here, but it may be mentioned that a river carries the 

 greater amount of its burden in times of flood, and that those are the 

 times when it also overspreads its flood-plain. From the broad and 

 shallow flood waters much material is thrown down, more being 



I " On the Lower Portion of the River Indus," Journal 0} the Geographical Society, 

 Vol. XXXVII (1867), p. 77. 



= "Marine Deposits in the Indian, Southern, and Antarctic Oceans," Scottish 

 Geographical Magazine, Vol. V, No. 8 (August, 1889), p. 420. 



