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The Lake Superior Lowland of Wisconsin
The Lake Superior LowlandOne of the most striking features of Lake Superior, the largest body of fresh water in the world, is its steeply- rising walls. Bounded by steep escarpments, it is a lake set deeply in a highland.
On the southwest is the Bayfield Peninsula and Douglas Copper Range of Wisconsin, 400 to 600 feet above the surface of the lake. On the south are the Penokee- Gogebic Iron Range, the Porcupine Mountains, and Keweenaw Point in Wisconsin and Michigan, 750 to 1400 above Lake Superior. On the northwest, north and northeast is the Lake Superior Highland of Minnesota and Canada, 800 to 1000 feet above the lake. It is only on the southeast, between Sault Ste Marie and Marquette, Michigan, that Lake Superior has a low- lying shore like the coast of Lake Michigan. This southeastern coast rises 300 feet or less above Lake Superior.
The portion of the basin lying at the western end of Lake Superior in Wisconsin and Minnesota is a graben, or rift valley. The history of the Lake Superior basis has been long and complicated. Its salient features may be summarized as follows:
Thus the Lake Superior basin is now a lowland because of the dropping down of a block of the earth's crust in a rift, or graben, fault. Subsequent sedimentation and erosion in the district are quite as important as the ancient faulting.
The Lake Superior Lowland in Wisconsin occupies portions of Douglas, Bayfield and Ashland counties in the northwestern corner of the state. Its area is about 1250 square miles, not including the 2400 square miles more in Wisconsin that is submerged beneath the waters of Lake Superior. Its altitude ranges from less than 1000 feet above to about 300 feet below sea level, and it rises 150 to 350 feet above and goes 600 to 900 feet below the level of Lake Superior, which stands 602 feet above sea level. In topography this province is chiefly a plain.
The escarpments which form the borders of this rift valley are probably of fault origin, but they have been much modified. The Superior escarpment in Wisconsin, though originally determined by a fault line, is now modified by the work of streams and glaciers to a gently- sloping wall. The short streams which flow down the face of the escarpment are in deep gorges and have rapids and waterfalls, as in the Bois Brule River of Douglas County and the Montreal River on the boundary between Wisconsin and Michigan. These gorges and waterfalls are postglacial features.
Sculpture by the continental ice sheet has notably modified the rift valley of northwestern Wisconsin. It is not clear whether Lake Superior existed before the first invasion by the continental glacier, but probably it did not. The preglacial master stream from the western end of the Lake Superior basin probably flowed eastward to some point between Marquette and Sault Ste Marie, Michigan. There it joined the Lake Michigan River and flowed southward, through what is now the basin of Lake Michigan to the Illinois and Mississippi rivers.
All of the scouring out of the buried rift valley below approximately the present level of Lake Superior was the work of the continental ice sheet. Glacial erosion certainly lowered the bottom of the rift valley to a depth of 600 to 900 feet. Its bottom now lies at levels between that of the surface of the ocean and 300 feet below sea level. In other parts of the rift valley the deepening was less, but everywhere there was profound glacial sculpture.
The Apostle Island are made up of comparatively weak sandstone and might be thought to represent hilltops between preglacial valleys. Glacial erosion seems to have deepened and widened channels between islands, to have eroded hilltops now island crests and to have modified the region sufficiently so that the preglacial drainage pattern is completely lost.
The retreating glacier deposited very little in the way of moraines in the Lake Superior Lowland. The chief visible deposit of this character is the kettle moraine in Bayfield County and its extension southward into Minnesota. there is also a narrow terminal moraine which reaches Lake Superior west of the Montreal River in Iron County.
Among the materials in the glacial drift of the Lake Superior Lowland are masses of native copper carried westward by the ice itself and in icebergs. One from the Bayfield Peninsula near La Pointe, Wisconsin weighs 800 pounds.
The plain south and west of Superior and south of Ashland, slopes gradually toward the lake, its grade varying from 10 to 50 feet to the mile. The surface, however, is not everywhere that of a plain, except near Superior and Ashland, where it furnishes the level sites of these cities. Farther from the lake it has been deeply trenched by postglacial streams, forming ravines 40 to 100 or more deep. The clay has been more extensively dissected than the sand because the water sinks into the latter and erodes it very little.
[A discussion of the drainages of the St Louis river, the Nemadji River, and the Bois Brule, Bad and Montreal rivers begins on page 439 of the book. Also in this section are discussions of Pattison and Copper Falls state parks.
[The Wisconsin Coast of Lake Superior, including submergence as an issue with implications for the state boundary, is discussed in detail beginning on page 446.
[Appendix B, pages 481-487, is an essay on the origins of the boundaries of the State of Wisconsin. Appendix F, pages 514-524, is an excellent overview of The Land Survey in Wisconsin.]
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