History’s Gem of the Month: Lake Superior Origin from 1957

August 2007

This article was contained in the museum’s archive. It was glued onto a page on which the museum founder, Axel Niemi, wrote that it was written in 1957 by Dr. George M Schwartz from the University of Minnesota. The source of publishing was not noted.

Origin of Lake Superior

Lake Superior, is the largest body of fresh water in the world and also one of the deepest. The term, superior, referring to its position as the upper lake of the five Great Lakes of North America was first used according to Grace Lee Nute in the Jesuit Relation of 1647-1648. The lake is 300 miles long and has a maximum width of 160 miles with an area of about 32,000 square miles. Its normal surface level is 602 feet above sea level and the greatest depth 1,290 feet.

In view of its great size and depth, it is natural that there should be much interest in the origin of the lake. In addition to its size, another fact of importance in considering the origin is occurrence of the lake in the trough of a great syncline or down fold in the rocks. That this structure of rocks has had an important effect on the origin of the lake is certain, but that the down folding of the rocks is the primary cause of the present lake is doubted inn view of the great length of time since the folding. The folding can be fairly closely dated as late Keweenawan or some 600,000 years ago. This would seem to allow plenty of time for any depression formed by the folding to have been filled with sediments.

Another fundamental fact to be considered in the origin of the present lake is the great depth of the bottom of the lake below sea level. The depression could therefore not have formed by stream erosion unless this whole portion of the continent once was much higher with respect to sea level than it is at present. While there have been changes in both continental and sea levels, there is no good evidence that these were on a scale to account for the Superior depression.

There are two main possible explanations: (1) The lake was scoured out by successive ice lobes which probably occupied the rock basin culminating in the Superior lobe of Late Wisconsin time. The glacial erosion was probably guided by a deep river valley in the structural basin. (2) Depression of the basin by faulting at a sufficiently late date for it to have escaped filling with sediment.

Difficulties with the fault hypothesis are the lack of evidence of late faulting anywhere at this portion of the North American continent and the lack of good evidence of faults in the proper positions to account for the depression. Certain great faults are known to exist in the Superior region and it is reasonable to assume that erosion was modified by their existence.

The hypothesis of glacial erosion has more obvious support. The movement of the Superior ice lobe down the axis of the lobe was one of the last great geologic events in the region. The timing to account for the present depression is excellent. Furthermore it is evident form the moraines of the Superior lobe as shown by Leverett’s maps that a great deal of rock debris was picked up by the ice lobe and deposited to the southwest of the present lake. It is also shown by glacial drift to the southwest in Minnesota that earlier ice sheets passed over the superior region and gathered up enormous amounts of rock debris and carried it on to the south. It is reasonable to suppose therefore that the Superior syncline was occupied by previous lobes that moved along the length of the basin gouging it out to a greater extent each time it was occupied by a lobe of ice.

In summary it may be said that Lake Superior probably owes its origin to a combination of conditions. The first important event was the formation of the great syncline following the extensive igneous extrusions and intrusions of the Keweenawan. This syncline no doubt was expressed at a surface by a basin that was filled by later and softer rocks than the older rocks round the edges. Faulting at a still later time modified the structure of portions of the syncline. Some of the faulting has been considered of late Keweenawan age, part of the movement is Paleozoic or later. The immediate cause of the present topographic basin was erosion by successive lobes of glacial ice that occupied the bottom of the syncline and eroded out the soft sediments but modified only in a moderate degree the resistant pre-Cambrian rocks on the sides. It is generally assumed that a large river valley occupied the present site of the lake and guided the early glacial erosion.

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