Topography.—As the surface of the continental shelf is ex tremely flat, so also is the land nearest the coast, which has but recently emerged and lies too near sea-level to favour the cutting of valleys. The landward edge of the coastal plain was necessar ily first to emerge from the sea. It is also the highest. For both of these reasons it is more deeply eroded and more completely dis sected by small streams than is the seaward edge. In the southern States this inner edge is at places more than 700 f t. above the sea and has a relief of 30o to 400 feet. At places erosion has even made new lowlands or peneplains. Other things equal, the num ber and depth of valleys increase inland.
Where the underlying strata resist erosion unequally, a strong stratum may give rise to a cuesta, a ridge with a very gentle slope down dip towards the sea and a steeper slope cutting across the strong stratum and leading down to an "inner lowland" on the underlying weak stratum. The effect on topography is to make the belted coastal plain wherein the stronger beds (usually sandy) make higher, rougher and less fertile belts and the weaker beds (usually clay, marl or "chalk") make belts of smooth, fertile lowland. Thus central Alabama has its well known "Black Belt" on the outcrop of the Selma chalk (Cretaceous) which erosion has reduced to a lowland with a rich black soil, famous for its cotton. Dipping seaward, the chalk passes under stronger sandy beds which are poor soil makers, hence the rich lowland is par alleled by a broad, now much dissected cuesta of poor upland. Another lowland and another upland follow but are less well known. Northern Texas has a similar belt, the "Black Prairie," along the western boundary of the coastal plain. It is likewise on a chalk outcrop and has on its seaward side an infertile cuesta. New Jersey has a similar lowland on the weak Raritan clays which outcrop along the landward edge of the coastal plain. From Trenton south-westward this low strip is followed by Delaware river. North-east from Trenton it is followed by the great rail roads running from New York to Philadelphia.
Islands.—It will be observed that the coastal plain is'broad est at the south and narrows toward the north. The opposite is true of the continental shelf, indicating that the continent is now relatively depressed at the north and elevated at the south. One effect of this is seen in the islands from New York to Cape Cod. These islands, Staten, Long, Block, Martha's Vineyard and Nan tucket, are disconnected fragments of the coastal plain, parts of one or more cuestas whose corresponding inner lowlands are sub merged. The low strip described across New Jersey is continuous
with Long Island sound. The same weak clays underlie both.
Coastline.—Throughout the Atlantic and Gulf coast recent (not necessarily current) sinking is in evidence. From New Eng land to Texas the smaller streams have estuaries, as the larger ones had also until filled by sediments. North of southern Vir ginia all the major streams crossing the coastal plain are drowned across its entire width. The numerous large estuaries from New York to North Carolina make that part of the coast extremely ragged or crenulated. Sand reefs or barrier beaches form a nearly straight "outer coast" and these continue most of the way from Long Island to Mexico. They are especially long and continuous along the coast of Texas where tides are weak.
The peninsula of Florida is due to an uplift of the sea bottom on an axis almost at right angles to the outline of the continent. Its southern third is so little above sea-level that valley cutting is all but impossible. Here are the vast Everglades (q.v.), swamps whose water surface is less than 20 f t. above the sea. Its most valuable part is a low limestone ridge on the Atlantic side, bearing Miami and other winter resorts. On this ridge and a line of coral reef wrapping around the southern end of the peninsula was the now abandoned Florida East Coast railroad. The northern part of the peninsula has suffered some erosion. Its underlying limestone suffers solution, giving rise to many sinkholes, some of them clogged and making lakes of considerable size. Here are the largest deposits of phosphate rock in eastern United States.