Of all the physical divisions of the continent they are at once the youngest and simplest in structure. They are floored with horizontal strata, whose sedimentary rocks were probably deposited on a deeply submerged portion of the Canadian Shield. It is possible to pass from the Arctic Ocean to the Gulf of Mexico without rising more than 1000 feet. The level or undulating character of their surface is unbroken, except in a few cases where uplands occur, such as the Black Hills of Dakota, and the Ozark Hills between the Missouri and Arkansas rivers.
The plains may be divided into
(a) the Northern Lowlands, between the Shield and the Rockies, and drained to the Arctic by the Mackenzie;
(b) the St. Lawrence-Great Lakes Lowlands;
(c) the Central Lowlands, drained by the Mississippi, whose elevation is nowhere more than 600 feet, and which merges into
(d) the Gulf Lowlands which with the southern part of the Atlantic costal plain, form the South-East Lowlands;
(e) the High Plains, stretching from the Central Lowlands towards the base of the Rockies
The Interior plains extend from the central Tennessee and Kentucky in the southeast and Texas and Oklahoma in the southwest northward into the Mackenzie valley or northern Canada. The area north of the Ohio and the Mississippi rivers has glacial landscape features such as till plains, glacial lakebeds and disarranged drainage. It is of low elevation (610 meters) and elevation gradually increases to the west (1219 - 1524 metre)
The Coastal plain borders Anglo-America from Cape Cod to Texas. The plain is of recent origin. Some parts of the coastal plains have the best soil resources of the area-the black prairie soils of Texas, Alabama and the Mississippi valley. The areas of poor drainage also exist as in the areas of the Dismal Swamp, the Okefenoker swamp and the Everglades.
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