Author: Karen

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.

July 2007

While looking through the museum’s archives to select this month’s history gem, I came across a draft of an article written by the museum founder, Axel Niemi. I’m not sure if and when he ever published this article, but I thought it worthy of sharing.

HINTS ON HUNTING GLACIAL AGATES

Grand Marais to Whitefish Point are gravelly for the most part. The glacial rocks leave the harder agates and other quartz minerals to be picked by the lucky hunter. There are a number of gravel pits scattered throughout the U.P., but the chances of finding anything of importance in these pits are slim. The author found only one large green jasper moss agate (1 pound), which was identical to a specimen found by a conservation worker on the high banks of the Sucker River. However, rockhounds will have better luck combing the beaches.

Many visitors examine the Gitche Gumee Mineral Museum collection every year and are amazed at the multiple varieties of agate, jasper, and other gem minerals that have come from the local beaches. That the collection was found within a few minutes walk to a couple hour drive from the museum seems unbelievable. To those rockhounds who understand the difficulty encountered in hunting these all but camouflaged mineral gemstones, it is apparent that a great deal of time is required to find these priceless rarities. Those who are willing to put in the time, though, are often rewarded.

The biggest reason beginners are not successful in finding glacial agates is the fact that they give up too easily. Since the agates have become worn to the point that they are rather indistinguishable from other translucent non-gem rocks and opaque stones, the problem of spotting a choice agate is readily understood. To add further to the confusion, until the rockhound becomes accustomed to spotting agate characteristics, it is easy for your focus to be diverted by a host of variously colored pretty jaspers, cherts, and granite pebbles that are thrown into the jumble. Even the experienced agate hunter must avoid being inflicted by “the pretty rock syndrome.”

Another reason why agates are hard to spot is that many are covered with dozens of tiny powdery fractures that may hide the true nature of the interior beauty. If you were more than a billion years old having to contend with the waves and ice of Lake Superior, as well as its predecessor lakes and oceans, you would have a few fractures, too. Thus to be successful it is important that you keep your focus, spend the hours required, and concentrate on looking for the agate characteristics. With such a variety of rocks on the beach, coupled with the need to look through or among millions of other glacial-worn stones, the task to spot the elusive agate is a challenge in itself.