Adirondacks; but to the south west the dissected highland continues
into Pennsylvania and Virginia, where it is commonly known as the
Alleghany plateau. A curious feature appears in northern Pennsylvania:
here the lateral pressure of the Palaeozoic mountain-making
forces extended its effects through a belt about fifty miles wider than
the folded belt of the Hudson Valley, thus compressing into great
rock waves a part of the heavy stratified series which in New York
lies horizontal and forms the Catskills; hence one sees, in passing
south-west from the horizontal to the folded strata, a beautiful
illustration of the manner in which land sculpture is controlled by
land structure. Altitudes of 1200 ft. prevail in Pennsylvania and
increase in Virginia; then the altitude falls to about 1000 ft. in
Kentucky and Tennessee, where the name Cumberland plateau is
used for the highest portion, and to still less in northern Alabama,
where the plateau, like the mountain belt, disappears under the
Gulf coastal plain. Through all this distance of 1000 m. the border
of the plateau on the south-east is an abrupt escarpment, eroded
where the folded structure of the mountain belt reveals a series of
weaker strata; but in the north-west the plateau suffers only a
gradual decrease of height and of relief, until the prairie plains are
reached in central Ohio and southern Indiana and Illinois, about
150 m. inland from the escarpment. Two qualifications must,
however, be added. In certain parts of the plateau there are narrow
anticlinal uplifts, an outlying effect of mountain-making compression;
here a ridge rises if the exposed strata are resistant, as in Chestnut
ridge of western Pennsylvania; but here a valley is excavated if the
exposed strata are weak, as in Sequatchie Valley, a long narrow
trough which cuts off a strip of the plateau from its greater body in
Tennessee. Again, in Kentucky and Tennessee, there is a double
alternation of sandstone and limestone in the plateau-making
strata; and as the skyline of the plateau bevels across these formations,
there are west-facing escarpments, made ragged by mature
dissection, as one passes from the topographically strong sandstone
to the topographically weak limestone.
In the north-east (New York and Pennsylvania) the higher parts of the plateau are drained by the Delaware and Susquehanna rivers directly to the Atlantic; farther west and south-west, the plateau is drained to the Ohio river and its branches. The submature or mature dissection of the plateau by its branching insequent streams results in giving it an excess of sloping surface, usually too steep for farming, and hence left for tree growth.
The Superior Oldland.—An outlying upland of the Laurentian highlands of Canada projects into the United States west and south of Lake Superior. Although composed chiefly of crystalline rocks, which are commonly associated with a rugged landscape, and although possessing a greatly deformed structure, which must at some ancient period have been associated with strong relief, the upland as a whole is gently rolling, and the inter-stream surfaces are prevailing plateau-like in their evenness, with altitudes of 1400 to 1600 ft. in their higher areas. In this province, therefore, we find a part of one of those ancient mountain regions, initiated by crustal deformation, but reduced by long continued erosion to a peneplain of modern relief, with occasional surmounting monadnocks of moderate height not completely consumed during the peneplanation of the rest of the surface. The erosion of the region must have been far advanced, perhaps practically completed, in very ancient times, for the even surface of the peneplain is overlapped by fossiliferous marine strata of early geological date (Cambrian); and this shows that a depression of the region beneath an ancient sea took place after a long existence as dry land. The extent of the submergence and the area over which the Palaeozoic strata were deposited are unknown; for in consequence of renewed elevation without deformation, erosion in later periods has stripped off an undetermined amount of the covering strata. The valleys by which the uplands are here and there trenched to moderate depth appear to be, in part at least, the work of streams that have been superposed upon the peneplain through the now removed cover of stratified rocks. Glaciation has strongly scoured away the deeply-weathered soils that presumably existed here in preglacial time, revealing firm and rugged ledges in the low hills and swells of the ground, and spreading an irregular drift cover over the lower parts, whereby the drainage is often much disordered; here being detained in lakes and swamps (“muskegs”) and there rushing down rocky rapids. The region is therefore generally unattractive to the farmer, but it is inviting to the lumberman and the miner.
The Adirondack Mountains.—This rugged district of northern New York may be treated as an outlier in the United States of the Laurentian highlands of Canada, from which it is separated by the St Lawrence Valley. It is of greater altitude (Mt Marcy 5344 ft.) and of much greater relief than the Superior Oldland; its heights decrease gradually to the north, west and south, where it is unconformably overlapped by Palaeozoic strata like those of Minnesota and Wisconsin; it is of more broken structure and form on the east, where the disturbances of the Appalachian system have developed ridges and valleys of linear trends, which are wanting or but faintly seen elsewhere. (See Adirondacks.)
Region of the Great Lakes.—The Palaeozoic strata, already mentioned as lapping on the southern slope of the Superior Oldland and around the western side of the Adirondacks are but parts of a great area of similar strata, hundreds of feet in thickness, which decline gently southward from the great oldland of the Laurentian highlands of eastern Canada. The strata are the deposits of an ancient sea, which in the earlier stage of geological investigation was thought to be part of the primeval ocean, while the Laurentian highlands were taken to be the first land that rose from the primeval waters. Inasmuch, however, as the floor on which the overlapping strata rest is, like the rest of the Laurentian and Superior Oldland, a worn-down mountain region, and as the lowest member of the sedimentary series usually contains pebbles of the oldland rocks, the better interpretation of the relation between the two is that the visible oldland area of to-day is but a small part of the primeval continent, the remainder of which is still buried under the Palaeozoic cover; and that the visible oldland, far from being the first part of the continent to rise from the primeval ocean, was the last part of the primeval continent to sink under the advancing Palaeozoic seas. When the oldland and its overlap of stratified deposits were elevated again, the overlapping strata must have had the appearance of a coastal plain; but that was long ago; the strata have since then been much eroded, and to-day possess neither the area nor the smooth form of their initial extent. Hence this district may be placed in the class of ancient coastal plains. As is always the case in the broad denudation of the gently inclined strata of such plains, the weaker layers are worn down in sub-parallel belts of lower land between the oldland and the belts of more resistant strata, which rise in uplands.
Few better illustrations of this class of forms are to be found than that presented in the district of the Great Lakes. The chief upland belt or cuesta is formed by the firm Niagara limestone, which takes its name from the gorge and falls out through the upland by the Niagara river. As in all such forms, the Niagara cuesta as a relatively strong slope or infacing escarpment on the side towards the oldland, and a long gentle slope on the other side. Its relief is seldom more than 200 or 300 ft., and is commonly of small measure, but its continuity and its contrast with the associated lowlands worn on the underlying and overlying weak strata suffice to make it a feature of importance. The cuesta would be straight from east and west if the slant of the strata were uniformly to the south; but the strata are somewhat warped, and hence the course of the cuesta is strongly convex to the north in the middle, gently convex to the south at either end. The cuesta begins where its determining limestone begins, in west-central New York; there it separates the lowlands that contain the basins of lakes Ontario and Erie; thence it curves to the north-west through the province of Ontario to the belt of islands that divides Georgian Bay from Lake Huron; then westward through the land-arm between lakes Superior and Michigan, and south-westward into the narrow points that divide Green Bay from Lake Michigan, and at last westward to fade away again with the thinning out of the limestone; it is hardly traceable across the Mississippi river. The arrangement of the Great Lakes is thus seen to be closely sympathetic with the course of the lowlands worn on the two belts of weaker strata on either side of the Niagara cuesta; Ontario, Georgian Bay and Green Bay occupy depressions in the lowland on the inner side of the cuesta; Erie, Huron and Michigan lie in depressions in the lowland on the outer side. When the two lowlands are traced eastward they become confluent after the Niagara limestone has faded away in central New York, and the single lowland is continued under the name of Mohawk Valley, an east-west longitudinal depression that has been eroded on a belt of relatively weak strata between the resistant crystalline rocks of the Adirondacks on the north and the northern escarpment of the Appalachian plateau (Catskills-Helderbergs) on the south; forming a pathway of great historic and economic importance between the Atlantic seaports and the interior.
In Wisconsin the inner lowland presents an interesting feature in a knob of resistant quartzite's, known as Baraboo Ridge, rising from the buried oldland floor through the partly denuded cover
