The platform and plateau.  Why do they stand at a common level ab?  Of what surface may they be remnants?  Is it accordant with the rock structure?  How was it produced?  What unconsumed masses overlooked it?  Did the rocks of the Appalachian valley stand above this surface when it was produced?  Did they then stand below it?  Compare the time needed to develop this surface with that needed to develop cd.  Which surface is the older?

How many cycles of erosion are represented here?  Give the erosion history of the region by cycles, beginning with the oldest, the work done in each and the work left undone, what brought each cycle to a close, and how long relatively it continued.

CHAPTER IV

RIVER DEPOSITS

The characteristic features of river deposits and the forms which they assume may be treated under three heads:  (1) valley deposits, (2) basin deposits, and (3) deltas.

VALLEY DEPOSITS

Flood plains are the surfaces of the alluvial deposits which streams build along their courses at times of flood.  A swift current then sweeps along the channel, while a shallow sheet of water moves slowly over the flood plain, spreading upon it a thin layer of sediment.  It has been estimated that each inundation of the Nile leaves a layer of fertilizing silt three hundredths of an inch thick over the flood plain of Egypt.

Flood plains may consist of a thin spread of alluvium over the flat rock floor of a valley which is being widened by the lateral erosion of a graded stream (Fig. 60).  Flood-plain deposits of great thickness may be built by aggrading rivers even in valleys whose rock floors have never been thus widened.

A cross section of a flood plain shows that it is highest next the river, sloping gradually thence to the valley sides.  These wide natural embankments are due to the fact that the river deposit is heavier near the bank, where the velocity of the silt-laden channel current is first checked by contact with the slower-moving overflow.

Thus banked off from the stream, the outer portions of a flood plain are often ill-drained and swampy, and here vegetal deposits, such as peat, may be interbedded with river silts.

A map of a wide flood plain, such as that of the Mississippi or the Missouri (Fig. 77), shows that the courses of the tributaries on entering it are deflected downstream.  Why?

The aggrading streams by which flood plains are constructed gradually build their immediate banks and beds to higher and higher levels, and therefore find it easy at times of great floods to break their natural embankments and take new courses over the plain.  In this way they aggrade each portion of it in turn by means of their shifting channels,