LARGER RELATIONS OF DELTAS 889 



have a grade of about 26 per 1,000, a slope of 1 degree 30 minutes; steep 

 in comparison with the other parts of the delta, but flat in comparison 

 with angles generated by material rolling or sliding down a slope. Be- 

 low the foreset slope and mainly to the eastward is a wide bottom, much 

 flatter, and constituting the bottomset beds of the delta, between 1,000 

 and 2,000 meters deep. The convexity of the contours about the shore 

 face down to 1,000 meters shows that these under-water parts are really 

 a construction from the river-borne detritus and not the original profile 

 of the Mediterranean basin. The Nile delta has not only given to 

 modern tongues the word delta as a generic name, but all its parts are 

 seen to be displayed in regular development, as befits a genotype. In the 

 Netherlands, on the contrary, a confluent delta fringe has been built 

 facing the North Sea by several rivers. The heavy northwest storms 

 have constructed a strong shore face, behind which is a broad band of 

 tidal flats ?ind lagoons. The latter have been in part developed by a 

 movement of subsidence pronounced during the past 800 years. The 

 war of the inhabitants against the North Sea has prevented the inroads 

 which the ocean might otherwise have made as a result of the subsidence, 

 but has also banished a large part of the lagoons and swamps which 

 would naturally mark the country. Outside of the guarding reefs the 

 shore face descends to a depth of 10 and in places to 20 meters, beyond 

 which a broad subaqueous plain stretches out and forms the bottom of 

 the North Sea, the 50-meter contour lying 300 kilometers distant from 

 the shore except over limited areas which are subject to excessive scour. 

 Over this bottom the sediment is moved chiefly by wave action; it does 

 not settle quietly from suspension, and the delta of the Netherlands can 

 not be said to have either foreset or bottomset beds. Although the sub- 

 sidence which has brought the North Sea into existence is known to be 

 geologically recent, the land and bottom profiles show that sedimentation 

 has already brought the subaerial and subaqueous plains into normal rela- 

 tion to each other. Erosion although dominant against the British coast 

 can have had but minor effect in the vicinity of the Netherlands. 



VARIATIONS AND BLENDING IN THE COMPONENT PARTS OF DELTAS 



The relative importance of the several components of a delta varies in 

 different examples within wide limits. First, fine waste and strong ocean 

 currents result in an extended development of the bottomset beds ; second, 

 deep quiet waters of constant level permit a large proportion of the waste 

 to accumulate as foreset beds ; third, strong wave action, breadth of delta 

 front, fineness of waste, and crustal subsidence favor the development of 

 a wide subaqueous plain; fourth, weak waves, coarse and abundant sedi- 



