The Atlantic and Gulf Coastal Plain throughout most of its extent in the United States is a sharply defined geologic province, but physiographically the province is not so sharply defined. The length of the plain from Cape Cod, Massachusetts, to the Rio Grande is about 2,200 miles. The breadth of the Atlantic portion of the plain (exclusive of Florida) averages about 100 miles; the breadth of the Gulf portion that lies within the United States (exclusive of the Mississippi Valley) averages about 250 miles. The Mississippi Valley from the Delta to Cairo is about 575 miles long and the peninsula of Florida is about 400 miles long.
This vast so-called plain rises from sealevel on the coast to a height of more than 1,000 feet in Texas, and its inner margin rises from a little more than 100 feet in places on Cape Cod (disregarding hills . . .