As the submarine delta grows near to the level of the sea the distributaries of the river cover it with subaërial deposits altogether similar to those of the flood plain, of which indeed the subaërial delta is the prolongation. Here extended deposits of peat may accumulate in swamps, and the remains of land and fresh-water animals and plants swept down by the stream are imbedded in the silts laid at times of flood. Borings made in the deltas of great rivers such as the Mississippi, the Ganges, and the Nile, show that the subaërial portion often reaches a surprising thickness. Layers of peat, old soils, and forest grounds with the stumps of trees are discovered hundreds of feet below sea level. In the Nile delta some eight layers of coarse gravel were found interbedded with river silts, and in the Ganges delta at Calcutta a boring nearly five hundred feet in depth stopped in such a layer. The Mississippi has built a delta of twelve thousand three hundred square miles, and is pushing the natural embankments of its chief distributaries into the Gulf at a maximum rate of a mile in sixteen years. Muddy shoals surround its front, shallow lakes, e.g. lakes Pontchartrain and Borgne, are formed between the growing delta and the old shore line, and elongate lakes and swamps are inclosed between the natural embankments of the distributaries. The delta of the Indus River, India, lies so low along shore that a broad tract of country is overflowed by the highest tides. The submarine portion of the delta has been built to near sea level over so wide a belt offshore that in many places large vessels cannot come even within sight of land because of the shallow water. [Illustration: Fig. 86. Radial Section of a Delta This section of a delta illustrates the structure of the platform which swift streams well loaded with coarse waste build in the water bodies into which they empty. Three members may be distinguished: the _bottom set beds_, _a_: the _fore set beds_, _b_; and the _top set beds_, _c_. Account for the slope of each of these. Why are the bottom set beds of the finer material and why do they extend beyond the others? How does the profile of this delta differ from that of an alluvial cone and why?] A former arm of the sea, the Rann of Cutch, adjoining the delta on the east has been silted up and is now an immense barren flat of sandy mud two hundred miles in length and one hundred miles in greatest breadth. Each summer it is flooded with salt water when the sea is brought in by strong southwesterly monsoon winds, and the climate during the remainder of the year is hot and dry. By the evaporation of sea water the soil is thus left so salty that no vegetation can grow upon it, and in places beds of salt several feet in thickness have accumulated. Under like conditions salt beds of great thickness have been formed in the past and are now found buried among the deposits of ancient deltas.