American geography

Region of the Great Lakes

The arrangement of the Great Lakes is 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 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 the 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 St 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 Mackinac Straits 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 ft. (160 ft. at the cataract) in 30 miles (50 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 Niagary 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.