the United States of America

Interior Low Plateaux.

West of the southern part of the Appalachian plateaux is a lower plateau province, mainly in the States of Kentucky and Tennessee. At its eastern edge it abuts against the western escarpment of the Cumberland plateau. Here the lower plateau is i,000 to 5,400 ft. high and scored by valleys between which the surface is little eroded. The ragged escarp ment of the Cumberland plateau rises Soo ft. higher. Everywhere the level of the Interior Low plateau declines toward Ohio river, near which in western Kentucky the uplands are little more than 500 ft. high. Dissection by streams increases toward the west end where it is complete. The upland level in most of the province, , except in the north-west, is approximately that of the Lexington (Highland Rim) peneplain which covers much of the continental interior. It is developed here on Mississippian (Lower Carbonif erous) rocks. Two low structural domes were thus truncated, the Nashville in Tennessee and the Cincinnati anticline, partly in Kentucky. Ordovician (Lower Silurian) limestones were ex posed in each, affording excellent soils. In Tennessee these soluble rocks were later cut down Soo ft. making the fertile Nashville basin, surrounded by steep slopes rising to the rela tively barren Highland Rim. The "Bluegrass Region" in Ken tucky preserves, in the main, the general level of the peneplain but is overlooked on the east by the Cumberland plateau, and on the south by a narrow monoclinal rim remaining from the former dome. In the central Bluegrass district are exposed the oldest rocks and the best soil-makers of the Cincinnati anticline. Western Kentucky embraces the south end of a great coal field with its centre in Illinois. Here is a broad, faint syncline in which the upper Carboniferous rocks were sufficiently depressed to escape the erosion that stripped them away from neighbouring areas. Here again, in western Kentucky, the soil is better.

Central Lowland and Superior Upland.

The term low land is not capable of exact definition. As applied to north central United States it signifies that most of the area is not obviously much above the level to which streams can cut down.

Within this great province, roughly one-fourth of the United States, the chief contrasts in topography are determined by glaciation. The entire province may be divided into six sections. Two of them were never covered by the ice; two of them were left after glaciation as relatively smooth till plains without lakes; two others were left with abundant moraines and undrained basins. Referring to the accompanying map and table, the Great Lake section (12a) has a topography controlled in detail by its thick glacial deposits, though its large features betray the relief of the underlying rock as carved by pre-glacial drainage. Termi nal and recessional moraines and outwash deposits, with morainic lakes and swamps, cover perhaps half of the area; the other half is till plains (ground moraine) and lacustrine plains. The beds of all the Great Lakes, except Superior, follow belts of weak strata which were presumably lowlands in pre-glacial time and were deepened by glacial erosion. As the ice front retreated northward from the Mississippi-St. Lawrence divide, lakes formed between the divide and the ice. With further retreat of the ice these lakes

expanded and merged, changing from time to time in shape, alti tude and outlet but generally discharging into the Mississippi system. When the vanishing ice covered only the St. Lawrence the lakes discharged for a time through Mohawk valley to the Hudson. As each newly discovered outlet lowered the lake levels the water was progressively withdrawn from the farther ends of the basins and the old lake bottoms at the heads of the lakes (south or west ends) were laid bare. An important though minor part of this Great Lake section consists of these perfectly flat former lake bottoms. The glaciation of this section was in the last, or Wisconsin, ice epoch and the surface has been little altered by erosion. Lakes and swamps are beginning to disappear but stream systems are poorly developed. West of Lake Michigan, in this section, is the leading dairy district of the United States.

The Western Young Drift 02/0 in Minnesota, northern Iowa and the Dakotas is similar but with a smaller proportion of mo rainic topography. It has also a smaller rainfall, so that forests are limited to northern Minnesota. An important feature of this section is the great lacustrine plain of former Lake Agassiz in the valley of Red river. This valley was flooded when the re treating ice cap obstructed the drainage to Hudson bay. The lacustrine plain (most of it in Canada) is now the greatest area of spring wheat production in America.

The two sections already described are separated by the Supe rior Upland and the Wisconsin Driftless Area. The former, as com pared with the other sections, is higher, is less deeply covered with drift, has a larger number of lakes and vastly more swamp. Its rocks are mainly igneous and metamorphic pre-Cambrian. Topo graphically it is a peneplain, upraised, somewhat eroded, then gla ciated. This is an important part of the great area of northern pine forest. Much is now cut-over land, only a minor part being valuable for agriculture. It contains the chief copper mines of eastern United States and the greatest iron mines of the world.

South-western Wisconsin and small parts of adjacent States were missed by the several ice invasions though surrounded by a glaciated surface that extends 25o m. farther south. Probably at no one time did the ice close around it. The margin of the ice sheet was divided into lobes following the great valleys. In the different ice epochs they came from different directions, at one time from the north-west, at another from the north-east, the areas covered by these different advances overlapping south of this Driftless Area. The southern and western parts of this area are limestone uplands rather deeply and at places sharply carved by erosion. The northern part is mainly a lowland underlain by friable Cam brian sandstone covered by glacial outwash. With due allowance for the effects of different kinds of rocks on topography, this driftless area may be regarded as a sample of the topography that was elsewhere overridden by the ice. It is not very different from that which is seen beyond the limit of glaciation in eastern Ohio or western Kentucky or central Missouri.