The Geology of Duluth Harbor EarthCache
The Geology of Duluth Harbor
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The difficulty level of this EarthCache is a 2.5 because it involves multiple locations. The terrain is probably a 1, but because of the construction at the Thompson Overlook Rest Stop, getting to the marker right now is NOT wheelchair accessible.
The Mid-continent Rift System or Keweenawan Rift is a 2,000 km (1,243 mi) long geological rift in the center of the North American continent and south-central part of the North American plate. It formed when the continent's core, the North American craton, began to split apart during the Mesoproterozoic era of the Precambrian, about 1.1 billion years ago. The rift failed, leaving behind thick layers of rock that are exposed on the surface in the Keweenaw Peninsula of Upper Michigan, northwest Wisconsin, and on the North Shore of the lake in Minnesota and Ontario and as far south as Interstate Park near Taylor’s Falls, Minnesota. Along most of its western and eastern arms the rift is buried beneath more recent sedimentary rocks up to 9 km (6 mi) thick.
Those arms meet at Lake Superior, which is contained within the rift valley. The lake's north shore in Ontario and Minnesota defines the northern arc of the rift. From the lake, the rift's eastern arm
trends south to central Lower Michigan. The western arm runs from Lake Superior southwest through portions of Wisconsin, Minnesota, Iowa, and Nebraska to northeastern Kansas.
Now lets fast-forward a billion years to the Cenozoic Era. Several glacial periods have covered this area with mile thick layers of slowly moving ice, advancing as far south as Missouri and Kansas. About 10,000 years ago, the Earth’s global temperatures increased and “global warming” caused the glaciers to melt. In front of this melting Superior glacial lobe the Glacial Lake Duluth formed. The glacial meltwater collected in this areas basin created a massive lake with a shoreline almost at the level of this geological marker. When the lake finally broke through its eastern dam, it drained the lake 197 feet (60 meters) below its present level. This dam busting created the current drainage of water through the Great Lakes, the St. Lawrence Seaway and finally to the Atlantic Ocean and the St. Croix River also drained some.
Waypoint 1- Geology of Duluth Harbor at Thompson Hill Overlook. These are the posted coordinates for the EarthCache
Waypoint 2- Canal marker Canal Park on the Visitors Center.
N 46. 46.789
W 092. 05.527
To receive credit for this earthcache, the following must be completed and emailed to me:
1. What two things are mentioned as forming the Duluth Harbor?
2. Find the elevation of the Geology of Duluth Harbor marker. Then go to Waypoint 2 in Canal Park (where you can read how humans changed “The Geology of Duluth’s Harbor” and find the elevation there. Adjust Waypoint 2’s elevation to what you think the water level elevation is in the Canal and subtract. Then subtract an additional 197 feet (60 meters) to calculate the surface water level after Glacial Lake Duluth drained from its original shoreline.
3. Posting a photograph of yourself or GPS at this site is now an optional requirement. Posting interesting photographs of this geological site (Canal Park or the Thompson Hill Overlook) to educate other visitors to this EarthCache and to document your experience here are appreciated.
To visit more EarthCaches about the Mid-continental Rift:
• Sandstone Lava Cake (GC1BB6X)
• Lake Superior: Fire and Ice (GC19AX3)
• IATCC- Ice Age Trail (GC1Z3Y6)
• Jay Cooke State Park (GC1BBMK)
• It’s Hudson’s Fault (GC235YP)
Email your answers to the questions, to me, using the link in my profile only. If your answers are not recieved by me in an appropriate amount of time, your log will be deleted. Photos are accepted and appreciated as long as the answers to the questions are not revealed. You do not have to wait for confirmation from me before logging this cache as completed. Most of all……learn……and enjoy the view.
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