Geography of The Interior United States - Region of The Great Lakes

Region of The Great Lakes

The Palaeozoic strata, already mentioned as lapping on the southern slope of the Superior Upland and around the western side of the Adirondack Mountains, are but parts of a great area of similar strata hundreds of feet in thickness. These strata decline gently southward from the great upland of the Laurentian Highlands of eastern Canada. The visible upland area of today was but a small part of the primeval continent with the remainder of it still buried under a Palaeozoic cover. The visible part was the last part of the primeval continent to sink under the advancing Palaeozoic seas. When the upland and its overlap of stratified deposits were elevated again, the overlapping strata must have had the appearance of a coastal plain. Of course that was long ago; since then the strata have eroded substantially and today possess neither the area nor the smooth form of their initial extent. This district may be considered an ancient coastal plain. 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 upland and the belts of more resistant strata, which rise in uplands.

Few better illustrations of this type of forms are to be found than that presented in the district of the Great Lakes. The chief upland belt or escarpment is formed by the firm Niagara limestone/dolostone, which takes its name from the gorge and falls cut through the upland by the Niagara River. As in all such forms, the Niagara Escarpment has a relatively strong slope or infacing escarpment on the side towards the upland, and a long gentle slope on the other side. Its relief is seldom more than 200 or 300 feet (91 m) and is generally small. Its continuity and its contrast with the associated lowlands on the underlying and overlying weak strata suffice to make it an important feature. The escarpment would lie straight east-west if the slant of the strata were uniformly to the south. However, the strata are somewhat warped and so the escarpment's course is strongly convex to the north in the middle, gently convex to the south at either end.

The escarpment begins where its determining limestone/dolostone begins, in west-central New York. There, it separates the lowlands that contain Lake Ontario from Lake Erie. It curves to the northwest through the Ontario province to the island belt that divides the Georgian Bay from Lake Huron. From there, it heads westward through the land-arm between Lake Superior and Lake Michigan and southwestward into the narrow points dividing Green Bay from Lake Michigan. Finally, it fades away with the thinning out of the limestone and is hardly traceable across the Mississippi River.

The arrangement of the Great Lakes closely matches the course of the lowlands worn on the two belts of weaker strata on either side of the Niagara escarpment. Lake Ontario, Georgian Bay and Green Bay occupy depressions in the lowland on the inner side of the escarpment. Lake Erie, Lake Huron and Lake 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 the Mohawk Valley. This is 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. Early in U.S. history, this provided a vital economic route between the Atlantic seaports and the U.S. interior.

In Wisconsin, the inner lowland has an interesting feature. It is a knob of resistant quartzites, known as Baraboo Ridge, rising from the buried upland floor through the partly denuded cover of lower Palaeozoic strata. This knob or ridge can be thought of as an ancient physiographic fossil, as it is an ancient monadnock having been preserved from destructive attacks of weather by burial under sea-floor deposits. It has been recently re-exposed through the erosion of its cover.

The occurrence of the lake basins in the lowland belts on either side of the Niagara escarpment is an abnormal feature. Ordinary erosion does not explain it. Glacial erosion has formed them through the glacial drift obstructing the normal outlet valleys and to crustal warping in connection with or independent of the glacial sheet.

Lake Superior is unlike the other lakes. The greater part of its basin occupies a depression in the upland area, independent of the overlap of Palaeozoic strata. The western half of the basin occupies a trough of synclinal structure. 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 Saint Lawrence on the north and to the Gulf of Mexico on the south.

The three lakes of the middle group stand at practically the same level:

• Lake Michigan
• Lake Huron
• Lake Erie

Lake Michigan and Lake Huron are connected by the Straits of Mackinac with the Mackinac Bridge spanning the straits. Lake Huron and Lake Erie are connected by the St. Clair River and Detroit River, with the small Lake St. Clair between them. The land northeast of the rivers is undergoing a slow elevation. The Niagara River connecting Lake Erie and Lake Ontario, with a fall of 326 feet (99 m) (160 feet (49 m) at the cataract) in 30 miles (48 km), is of very recent origin, as an older river would have a mature valley. The original valley that is thought to have connected the two depressions through the Niagara Escarpment is thought to have been at the present route of the Welland Canal, and to have been completely filled with glacial drift. The same is true for the St. Lawrence, where there may not have been an original valley. The Ontarian River that was a precursor to Lake Ontario is thought to have drained westward, and the St. Lawrence drainage to have been created by subsidence due to the weight of the ice sheet.