the subaerial 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 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.