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“Bite-Size Bits of Local, National, and Global History”
“Bite-Size Bits of Local, National, and Global History”
Near Proctor in St. Louis County, Minnesota — The American Midwest (Upper Plains)
 

Geology of Duluth Harbor

 
 
Geology of Duluth Harbor Marker image. Click for full size.
By Keith L, September 26, 2007
1. Geology of Duluth Harbor Marker
Inscription. Lake Superior is situated over the Midcontinent Rift, which is a rupture in the North American continent that formed a great rift valley from the Lake Superior region southwest to Kansas about 1.1 billion years ago. For about 20 million years as the rift valley opened, basaltic lavas erupted into it, accumulating to a thickness of up to 20 kilometers in the Lake Superior region. After the rifting and volcanic activity ended, the great thickness of dense basalt here depressed the crust into a trough-shaped basin. As the depression formed, it was filled in by sediment eroded and washed in from the surrounding heights. Ultimately, the sedimentary deposits reached a thickness of many kilometers.

Over the past two million years, glaciers more than a kilometer thick have repeatedly advanced along the buried trough and scoured out much of the soft sedimentary rock that once filled it. The harder, erosion-resistant volcanic rocks along the margins of the trough now form the rocky coastline of much of Lake Superior.

Ten thousand years ago, as the glacial ice in the basin melted west to east, water ponded in front of the ice almost to the level of this marker to form Glacial Lake Duluth. Eventually, the ice melted out of the eastern lake basin and a drainage way opened to the lower Great Lakes. When the eastern outlet first
Geology of Duluth Harbor Marker image. Click for full size.
By Keith L, September 26, 2007
2. Geology of Duluth Harbor Marker
formed, it was lower in elevation than today and drained the lake to 60 meters below its present level.

Relieved of the great weight of this glacial ice, the earth's crust has been slowly rising. The rate of rebound is fastest where the load of ice has been most recently removed. Thus, the northeastern lake basin and its eastern outlet are rising faster, thereby tilting the basin toward the southwest and flooding the mouth of the St. Louis River. Duluth harbor, which was formed by this submergence, has been enlarged by the formation of Minnesota Point, a baymouth bar sand deposit washed there by easterly waves and shore currents transporting beach sand from Wisconsin.

Erected by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation and the Minnesota Geological Survey 1998
 
Erected 1998 by the Geological Society of Minnesota.
 
Location. 46° 43.757′ N, 92° 12.317′ W. Marker is near Proctor, Minnesota, in St. Louis County. Marker can be reached from Interstate 35 at milepost 249, on the left when traveling north. Click for map. Marker is in a rest area accessible from Interstate 35. Marker is in this post office area: Duluth MN 55810, United States of America.
 
Other nearby markers. At least 8 other markers are within 5 miles
Geology of Duluth Harbor Marker image. Click for full size.
By Keith L, September 26, 2007
3. Geology of Duluth Harbor Marker
of this marker, measured as the crow flies. Minnesota’s Seaport / Welcome to Minnesota (within shouting distance of this marker); The Gate (within shouting distance of this marker); Gonyea, Verville and National Guardsmen Memorial (approx. 1.2 miles away); Engine 225 (approx. 1.3 miles away); Oneota (approx. 3.4 miles away); Summer White House - 1928 (approx. 4.9 miles away in Wisconsin); Rice's Point (approx. 5 miles away); Enger Observation Tower (approx. 5 miles away). Click for a list of all markers in Proctor.
 
More about this marker. Marker is on a concrete pedestal by the back observation deck at the Thompson Hill Travel Information Center/Rest Area (about 7 miles southwest of downtown Duluth). Exit at South Boundary Avenue (County Road 14), to Westgate Boulevard, then to Thompson Hill Visitors Center Parkway.
 
Categories. Natural Features
 
Nearby Geodetic Marker image. Click for full size.
By Keith L, September 26, 2007
4. Nearby Geodetic Marker
Geodetic Survey (Benchmark) Disk image. Click for full size.
By Keith L, September 26, 2007
5. Geodetic Survey (Benchmark) Disk
 
 
Credits. This page originally submitted on , by Keith L of Wisconsin Rapids, Wisconsin. This page has been viewed 2,187 times since then and 27 times this year. Photos:   1, 2, 3, 4, 5. submitted on , by Keith L of Wisconsin Rapids, Wisconsin. • Kevin W. was the editor who published this page. This page was last revised on June 16, 2016.
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