Borings made in the deltas of great rivers such as the Mississippi, the Ganges, and the Nile, show that the subaerial 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.

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.

Subsidence of great deltas.  As a rule great deltas are slowly sinking.  In some instances upbuilding by river deposits has gone on as rapidly as the region has subsided.  The entire thickness of the Ganges delta, for example, so far as it has been sounded, consists of deposits laid in open air.  In other cases interbedded limestones and other sedimentary rocks containing marine fossils prove that at times subsidence has gained on the upbuilding and the delta has been covered with the sea.

It is by gradual depression that delta deposits attain enormous thickness, and, being lowered beneath the level of the sea, are safely preserved from erosion until a movement of the earth’s crust in the opposite direction lifts them to form part of the land.  We shall read later in the hard rocks of our continent the records of such ancient deltas, and we shall not be surprised to find them as thick as are those now building at the mouths of great rivers.