The west border of the coastal plain from New York southward to central Georgia is at the junction of the soft, unconsolidated sands and clays of the emerged portion of the continental shelf, with hard and usually crystalline rocks of great geological age forming, an upland known as the Piedmont plateau, which extends westward to the base of the Appalachian Mountains. The sharply defined boundary between the plain and the plateau is termed the _fall line_, for the reason that it is marked by the lowest falls and rapids in the streams flowing eastward from the Appalachian Mountains. Throughout the courses of these streams to the west of the fall line they are shallow and swift and broken by many picturesque rapids, while to the east of the fall line they broaden in the soft sediments of the coastal plain, and are deep, placid streams which widen into estuaries. The influence of the tides is felt in these drowned rivers to the fall line. The most important fact in this connection is that the lower courses of the larger rivers, such as the Delaware, Susquehanna, Potomac, James, etc., are navigable for ocean-going vessels, while their upper courses to the west of the fall line are difficult to traverse even in canoes. [Illustration: FIG. 14.--Relief map of North America. After United States Geological Survey and Canadian Geological Survey.] The fall line is thus the head of navigation in a number of rivers, and for this reason it has determined the sites of several important cities. Its course is marked by Trenton, Philadelphia, Baltimore, Washington, Richmond, Weldon, Raleigh, Augusta, and Macon. Farther south, about the landward margin of the portion of the coastal plain bordering the Gulf of Mexico, the fall line is less distinct, largely for the reason that the rocks bordering it on the north and west are less resistant than those forming the plateau at the east base of the Appalachians. Exceptions to the fact that the coastal plain is composed mostly of soft sediments occur in southern Florida and in Yucatan, where coral rock has been upraised. Southeastward from Yucatan a coastal plain is wanting and rocky bluffs separated by stream-cut valleys come boldly down to the surf line. Partially drowned valleys on each side of Central America bear record of a recent but moderate downward movement of the land. From New York northward along the border of the continent the coastal plain is mostly lacking, or if recognisable, is greatly modified by glacial deposits, and the Piedmont plateau, as it is known farther south, swings eastward and becomes a coastal plateau with a more or less roughened surface, which extends northward to Labrador and the Arctic Ocean.