6i8 PRINCIPLES OF STRATIGRAPHY 



be true in regions past maturity and of slight supply of waste. 

 It is thus readily conceivable that periods of intensive and extensive 

 delta building may alternate in the earth's history with periods 

 during which deltas are of relatively insignificant extent and the 

 size and number of deltas in one period afford no criterion by which 

 those of another may be measured. 



Effect of subsidence. While the effect of stationary sea-level 

 will be the rapid outward building of the delta, with pronounced 

 development of the fore-set beds, slow subsidence will result in 

 the extensive development of top-set beds and a restriction in the 

 building of the fore-set beds. If the subsidence is periodic and 

 interrupted, the sea will temporarily encroach on the delta, and 

 the terrestrial top-set beds will be covered by a layer of marine 

 sediments. Continued deposition by the rivers will, however, crowd 

 back the sea, and renewed building of top-set beds will occur. In 

 this manner intercalated marine layers will be formed between the 

 terrestrial deposits. Such layers are found in modern deltas as 

 already noted, and they are also common in older delta deposits, 

 where they are generally taken as indicating the marine origin of 

 the entire formation. That they have no such value is clearly 

 shown by their relation to the continental beds in modern delta 

 deposits. If subsidence were slow and more or less regular, con- 

 tinued deposition by rivers would tend to keep the sea out altogether, 

 and a thick deposit of top-set beds of terrestrial origin would result. 

 Such deposits of considerable thickness are found to characterize 

 the larger modern deltas, which thus appear to occupy sites of 

 continued subsidence. Similar conditions prevailed over the sites 

 of delta deposits of IMesozoic and Palaeozoic time, resulting in the 

 formation of continental deposits of great thickness. 



Subsidence of such a rate as to be in excess of the building 

 power of the streams will put an end to the process of building the 

 deltas and result in their final submergence. This has been brought 

 about in a number of cases where former deltas and parts of 

 deltas are now below sea-level. In earlier geologic periods such 

 complete submergence has resulted in the burial of the delta de- 

 posit beneath a marine series of greater or less thickness. Credner 

 (14) and others have, indeed, held that any subsidence is detri- 

 mental to the process of delta building, and that such stuctures are 

 found only on rising or at least stationary coasts. That subsidence 

 has, however, played a part in the formation of modern deltas is 

 shown by the presence in most of them of terrestrial remains and 

 peat layers, etc., which now lie at a considerable depth beneath 

 sea-level (Fig. 127). 



