Geology of Duluth Harbor

Geology of Duluth Harbor (HM2UR)

Location: Duluth, MN 55810 St Louis County
Country: United States of America

N 46° 43.757', W 92° 12.317'

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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 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
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Details
HM NumberHM2UR
Tags
Year Placed1998
Placed ByThe Geological Society of Minnesota
Marker ConditionNo reports yet
Date Added Thursday, October 2nd, 2014 at 3:06pm PDT -07:00
Locationbig map
UTM (WGS84 Datum)15T E 560723 N 5175387
Decimal Degrees46.72928333, -92.20528333
Degrees and Decimal MinutesN 46° 43.757', W 92° 12.317'
Degrees, Minutes and Seconds46° 43' 45.42" N, 92° 12' 19.02" W
Driving DirectionsGoogle Maps
Area Code(s)218
Closest Postal AddressAt or near 8000-8254 W Skyline Pkwy, Duluth MN 55810, US
Alternative Maps Google Maps, MapQuest, Bing Maps, Yahoo Maps, MSR Maps, OpenCycleMap, MyTopo Maps, OpenStreetMap

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