Page:EB1911 - Volume 27.djvu/641

PHYSICAL GEOGRAPHY] of lower Palaeozoic strata. This knob or ridge may be appropriately regarded as an ancient physiographic fossil, inasmuch as, being a monadnock of very remote origin, it has long been preserved from the destructive attack of the weather by burial under sea-floor deposits, and recently laid bare, like ordinary organic fossils of much smaller size, by the removal of part of its cover by normal erosion.

The occurrence of the lake basins in the lowland belts on either side of the Niagara cuesta is an abnormal feature, not to be explained by ordinary erosion, which can produce only valleys. The basins have been variously ascribed to glacial erosion, to obstruction of normal outlet valleys by barriers of glacial drift, and to crustal warping in connexion with or independent of the presence of the glacial sheet. No satisfactory solution of this problem has been reached; but the association of the Great Lakes and other large lakes farther north in Canada with the great North American area of strong and repeated glaciation is highly suggestive.

Lake Superior is unlike the other lakes; the greater part of its basin occupies a depression in the oldland area, independent of the overlap of Palaeozoic strata. The western half of the basin occupies a trough of synclinal structure; but the making of this syncline is so ancient that it cannot be directly connected with the occurrence of the lake to-day. A more reasonable explanation ascribes the lake basin to a geologically modern depression within the Superior oldland area; but there is at present no direct evidence in favour of this hypothesis. The Great Lakes are peculiar in receiving the drainage of but a small peripheral land area, enclosed by an ill-defined water-parting from the rivers that run to Hudson Bay or the Gulf of St Lawrence on the north and to the Gulf of Mexico on the south.

Large canals and locks on both sides of the Sault (pronounced Soo) Ste Marie in the outlet of Lake Superior are actively used except during three or four winter months. The three lakes of the middle group stand at practically the same level: Michigan and Huron are connected by the Strait of Mackinac (pronounced Mackinaw); Huron and Erie by the St Clair and Detroit rivers, with the small Lake St Clair between them. The navigable depth of these two short rivers is believed to be the result of a slow elevation of the land in the north-east, still in progress, whereby the waters have risen on their former shores near Detroit. Niagara river, connecting lakes Erie and Ontario, with a fall of 326 ft. (160 ft. at the cataract) in 30 m., is manifestly a watercourse of very modern origin; for a large river would now have a thoroughly matured valley had it long followed its present course; the same is true of the St Lawrence, which in its several rapids and in its subdivision into many channels at the Thousand Islands, presents every sign of youth. Canals on the Canadian side of these unnavigable stretches admit vessels of a considerable size to lakes Ontario and Erie.

The Prairie States.—The originally treeless prairies of the upper Mississippi basin began in Indiana and extended westward and north-westward until they merged with the drier region described beyond as the Great Plains. An eastward extension of the same region, originally tree-covered, extended to central Ohio. Thus the prairies may be described as lying in a general way between the Ohio and Missouri rivers on the south and the Great Lakes on the north. Under the older-fashioned methods of treating physical geography, the prairies were empirically described as “level prairies,” “rolling prairies,” and so on. The great advance in the interpretation of land forms now makes it possible to introduce as thoroughly explanatory a description of these fertile plains as of forms earlier familiar, such as sand dunes, deltas and sea cliffs. The prairies are, in brief, a contribution of the glacial period; they consist for the most part of glacial drift, deposited unconformably on an underlying rock surface of moderate or small relief. The rocks here concerned are the extension of the same stratified Palaeozoic formations already described as occurring in the Appalachian region and around the Great Lakes. They are usually fine-textured limestones and shales, lying horizontal; the moderate or small relief that they were given by mature preglacial erosion is now buried under the drift, but is known by numerous borings for oil, gas and water.

The greatest area of the prairies, from Indiana to North Dakota, consists of till plains, that is, sheets of unstratified drift, 30, 50 or even 100 ft. thick, which cover the underlying rock surface for thousands of square miles (except where post glacial stream erosion has

locally laid it bare), and present an extraordinarily even surface. The till is presumably made in part of preglacial soils, but it is more largely composed of rock waste mechanically comminuted by the creeping ice sheets; although the crystalline rocks from Canada and some of the more resistant stratified rocks south of the Great Lakes occur as boulders and stones, a great part of the till has been crushed and ground to a clayey texture. The till plains, although sweeping in broad swells of slowly changing altitude, are often level to the eye, and the view across them stretches to the horizon, unless interrupted by groves of trees along the watercourses, or by belts of low morainic hills. Here and there faint depressions occur, occupied by marshy “sloughs,” or floored with a rich black soil of post-glacial origin. It is thus by sub-glacial aggradation that the prairies have been levelled up to a smooth surface, in contrast to the higher and non-glaciated hilly country next south.

The great ice sheets formed terminal moraines around their border at various halting stages; but the morainic belts are of small relief in comparison to the great area of the ice; they rise gently from the till plains to a height of 50, 100 or more feet; they may be one, two or three miles wide; and their hilly surface, dotted over with boulders, contains many small lakes in basins or hollows, instead of streams in valleys. The morainic belts are arranged in groups of concentric loops, convex southward, because the ice sheets advanced in lobes along the lowlands of the Great Lakes; neighbouring morainic loops join each other in re-entrants (north-pointing cusps), where two adjacent glacial lobes came together and formed their moraines in largest volume. The discovery of this significant looped arrangement of the morainic belts is the greatest advance in interpretation of glacial phenomena since the first suggestion of a glacial period; it is also the strongest proof that the ice here concerned was a continuous sheet of creeping land ice, and not a discontinuous series of floating icebergs, as had been supposed. The moraines are of too small relief to be shown on any maps but those of the largest scale; yet small as they are, they are the chief relief of the prairie states, and, in association with the nearly imperceptible slopes of the till plains, they determine the course of many streams and rivers, which as a whole are consequent upon the surface form of the glacial deposits.

The complexity of the glacial period and its subdivision into several glacial epochs, separated by interglacial epochs of considerable length (certainly longer than the post glacial epoch) has a structural consequence in the superposition of successive till sheets, alternating with non-glacial deposits, and also a physiographic consequence in the very different amount of normal post-glacial erosion suffered by the different parts of the glacial deposits. The southernmost drift sheets, as in southern Iowa and northern Missouri, have lost their initially plain surface and are now maturely dissected into gracefully rolling forms; here the valleys of even the small streams are well opened and graded, and marshes and lakes are wanting: hence these sheets are of early Pleistocene origin. Nearer the Great Lakes the till sheets are trenched only by the narrow valleys of the large streams; marshy sloughs still occupy the faint depressions in the till plains, and the associated moraines have abundant small lakes in their untrained hollows: hence these drift sheets are of late Pleistocene origin.

When the ice sheets fronted on land sloping southward to the Ohio, Mississippi and Missouri rivers, the drift-laden streams flowed freely away from the ice border; and as the streams, escaping from their sub glacial channels, spread in broader channels, they ordinarily could not carry forward all their load; hence they acted not as destructive but as constructive agents, and aggraded their courses. Thus local sheets or “aprons” of gravel and sand are spread more or less abundantly along the outer side of the morainic belts; and long trains of gravel and sands clog the valleys that lead southward from the glaciated to the non-glaciated area. Later when the ice retreated farther and the unloaded streams returned to their earlier degrading habit, they more or less completely scoured out the valley deposits, the remains of which are now seen in terraces on either side of the present flood plains.

When the ice of the last glacial epoch had retreated so far that its front lay on a northward slope, belonging to the drainage area of the Great Lakes, bodies of water accumulated in front of the ice margin, forming glacio-marginal lakes. The lakes were small at first, and each had its own outlet at the lowest depression in the height of land to the south; but as the ice melted back, neighbouring lakes became confluent at the level of the lowest outlet of the group; the outflowing streams grew in the same proportion and eroded a broad channel across the height of land and far down stream, while the lake waters built sand reefs or carved shore cliffs along their margin, and laid down sheets of clay on their floors. All of these features are easily recognized in the prairie region. The present site of Chicago was determined by an Indian portage or “carry” across the low divide between Lake Michigan and the headwaters of the Illinois river; and this divide lies on the floor of the former outlet channel of the glacial Lake Michigan. Corresponding outlets are known for the glacial lakes Erie, Huron and Superior, and for a very large sheet of water, named Lake Agassiz, which once overspread a broad till plain in northern Minnesota and North Dakota. The outlet of this glacial lake, called river Warren, eroded a large channel in which the Minnesota river, of to-day is an evident “misfit.”