West Duluth near Proctor in Saint Louis County, Minnesota — The American Midwest (Upper Plains)

Geology of Duluth Harbor

Photographed By Keith L, September 26, 2007

1. Geology of Duluth Harbor Marker

Inscription.

Geology of Duluth Harbor. . 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 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

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

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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.

Topics and series. This historical marker is listed in this topic list: Natural Features. In addition, it is included in the Minnesota: Geological Society of Minnesota series list. A significant historical year for this entry is 1998.

Location. 46° 43.757′ N, 92° 12.317′ W. Marker is near Proctor, Minnesota, in Saint Louis County. It is in West Duluth. Marker can be reached from Interstate 35 at milepost 249,

Photographed By Keith L, September 26, 2007

2. Geology of Duluth Harbor Marker

on the left when traveling north. Marker is in a rest area accessible from Interstate 35. Touch for map. Marker is in this post office area: Duluth MN 55810, United States of America. Touch for directions.

Other nearby markers. At least 8 other markers are within 5 miles 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); Rotary Club of Duluth International Service Projects (approx. 5 miles away). Touch for a list and map 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.

Photographed By Keith L, September 26, 2007

3. Geology of Duluth Harbor Marker

Nearby Geodetic Marker image. Click for full size.

Photographed By Keith L, September 26, 2007

4. Nearby Geodetic Marker

Geodetic Survey (Benchmark) Disk image. Click for full size.

Photographed By Keith L, September 26, 2007

5. Geodetic Survey (Benchmark) Disk

Credits. This page was last revised on February 12, 2023. It was originally submitted on February 3, 2008, by Keith L of Wisconsin Rapids, Wisconsin. This page has been viewed 2,819 times since then and 35 times this year. Photos: 1, 2, 3, 4, 5. submitted on February 3, 2008, by Keith L of Wisconsin Rapids, Wisconsin. • Kevin W. was the editor who published this page.