LARGER RELATIONS OF DELTAS 887 



and broad flood waters, and in arid or semi-arid climates the wind may 

 take its part, but the direction in which it moves the waste is not deter- 

 mined by that slope which governs tlie movement of water. Tt is tlie 

 subaerial plain whicli in i)opidar thought constitutes llic dcha, but it is 

 seen that in the process of delta-building it is but one facet of the largci- 

 surface of construction which reaches from the upper valley to the bottom 

 of the water body. The outer parts of the subaerial plain of larger deltas 

 are broadly flooded at longer or shorter intervals either from the river or 

 the sea, but the distinguishing feature is that the surface is periodically 

 exposed to the air, and marks of such exposure may be recorded in the 

 accumulating sediments. Lacustrine and lagoon deposits, ranging from 

 fresh to salt water conditions, are also intercalated, but the life of the 

 plain as a whole is more of the land than of the sea. 



The word delta is taken to refer more especially to the low-lying and 

 projecting portion of the river deposit, but this grades with no line of 

 demarcation into the materials of the river plain, which may reach far- 

 ther inland. In ancient formations all parts of a fluviatile deposit which 

 lie beyond the walls of the valley and face a body of permanent water 

 must be regarded as essentially parts of a delta. From this standpoint 

 the delta plain of China reaches westward more than 300 miles, and the 

 deltas of the Ganges and Indus can not be set apart, so far as their strata 

 are concerned, from the Indo-Gangetic floodplain, which completely 

 separates the peninsula of India from the Himalayan mountain system. 



The Nile and Rhine deltas as c.vamphs. — In order to give a propor- 

 tionate view of the several parts of deltas as seen in actual examples, 

 drawings are given of, first, the delta of the Xile, and, second, the com- 

 bined deltas of the Ehine and Meuse and that of the Ems. In both ex- 

 amples the shore face is pronounced ; strong wave action developing well 

 marked barrier beaches, behind which are shallow lagoons dotted with 

 islands and muddy flats. On ilie Xile deha the shore face extends to a 

 depth of 6 to 10 meters or thereabouts, as shown by the closeness of the 

 6-meter contour to the land and the considerable distance of the 20- 

 meter contour, not shown on tlie map. The 50-meter contour averages 

 about 50 kilometers from shore and is especially developed on the east 

 side of the delta, since this is the direction in which the dominant waves 

 and currents carry the bottom material. The slope of the subaqueous 

 plain is seen to be less than one in a thousand. From 50 to 200 meters 

 in a transition zone to the foreset slope, 50 kilometers wide in front of 

 the delta, but much luirrower on the sides. Below 200 meters waves 

 have no effect, and in a width of 30 kilometers there is a rapid descent 

 to 1,000 meters depth. This, then, is the foreset slope. Tt is seen to 



