5), representing the proportion of North America which is known to have been above water during the six geologic periods that intervene.
From this diagram it will appear that the six cycles or periods were by no means equal in the amount of overflow or complete recovery of the drowned lands.

The Cretacic period was marked by a much more extensive and long continued flooding; the great plains west of the Mississippi were mostly under water from the Gulf of Mexico to the Arctic Ocean.

The earlier overflows were neither so extensive nor so long continued.

The great uplift of the close of the Cretacic regained permanently the great central region and united East and West, and the overflows of the Age of Mammals were mostly limited to the South Atlantic and Gulf coasts.
_Sedimentary Formations._ During the epochs of greatest overflow great marine formations were deposited over large areas of what is now dry land.

These were followed as the land rose to sea level by extensive marsh and delta formations, and these in turn by scattered and fragmentary dry land deposits spread by rivers over their flood plains.