488 



CAUSES OF STRATIFICATION IN DELTAS. 



[Ch. XIX. 



littoral deltas first began to form. For this reason some of 

 the newer strata of sand or coarser materials are often found 

 to overlie an older set of much finer grain. By reflecting on 

 these facts, we see that the law of arrangement must be very 

 complex, more especially the relation between the relative 

 age of the different groups of sediment and the fineness of 

 their component materials. 



In those deltas where the tides and strong marine currents 

 interfere, the above description would only be applicable, with 

 certain modifications. If a series of earthquakes accompany 

 the growth of a delta, and change the levels of the land from 

 time to time, as in the region where the Indus now enters 

 the sea, the phenomena will depart still more widely from 

 the ordinary type. If, after a protracted period of rest, a 



shallow 



water near the foot of the boundary hills, so as to form con- 

 glomerates overlying the fine mud previously thrown into 



delta sink down, pebbles may be borne alon 



m 



deeper water in the same area. 



Causes of stratification in deltas. — The stratified arrange- 

 ment, which is observed to prevail so generally in aqueous 

 deposits, is most frequently due to variations in the velocity 

 of running water, which cannot sweep along particles of 

 more than a certain size and weight when moving at a given 

 rate. Hence, as the force of the stream augments or de- 

 creases, the materials thrown down in successive layers at 

 particular places are rudely sorted, according to their dimen- 

 sions, form, and specific gravity. Where this cause has not 

 operated, as where sand, mud, and fragments of rock are 

 conveyed by a glacier, a confused heap of rubbish devoid of 

 all stratification is produced. 



Natural divisions are also occasioned in deltas, by the in- 

 terval of time which separates annually the deposition of 

 matter during the periodical rains, or melting of the snow 

 upon the mountains. The deposit of each year may acquire 

 some degree of consistency before that of the succeeding 

 year is superimposed. A variety of circumstances also give 

 rise annually, or sometimes from day to day, to slight varia- 

 tions in colour, fineness of the particles, and other charac- 

 ters, by which alternations of strata distinct in texture, and 







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