One of the most typical portions of the fringe of swamps now bordering the land is situated in eastern Virginia and North Carolina, and is known as the Dismal Swamp. In the central portion of this marshy region, embracing some 700 square miles, lies Lake Drummond, an example of a large number of small fresh-water lakes which are retained by rims composed of plant growths and decaying vegetable matter. The mound of vegetable débris in the summit of which Lake Drummond is situated is from 20 to 30 miles broad and rises some 12 feet above tide-level. The lake is nearly circular, from 2 to 2½ miles in diameter, and from 6 to 10 feet deep. The water is amber-coloured on account of the vegetable matter in solution, but is clear and without sediment in suspension, and is considered as remarkably wholesome. The lake was without definite outlet previous to the cutting of drainage-canals,

and is entirely encircled by a dense forest, which has encroached on its border in such a manner as to render its boundaries indefinite. The wall of rank vegetation surrounding the open waters of the lake marks the beginning of the encircling swamp. Standing in the lake and supported by their widely expanded roots are several aged cypress-trees.

Along the coast of the Carolinas and Georgia sand-bars thrown up by the sea have formed many lagoons (Fig. 6), which are being filled by the wash of detritus from the land, by sand blown from their confining ridges, and by vegetation and the hard parts of molluscs, crustaceans, etc., living in their waters. In part, these areas have been converted into swamps, and are gradually being transformed into dry land. Farther southward, about the shores of Florida, and thence along the Gulf border, the low, indefinite margin of the coastal plain is fringed in many places by dense thickets of mangrove-trees, which extend their aerial roots into the salt water, and by retaining sediment and dead vegetation as well as by furnishing conditions favourable for animal life, lead to a gradual extension of the land.

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.

Fig. 14.—Relief map of North America. After United States Geological Survey and Canadian Geological Survey. [Larger NW Quadrant View.]
[Larger NE Quadrant View.]
[Larger SW Quadrant View.]
[Larger SE Quadrant View.]

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.