This Earthcache is located within Grand Portage National Monument. Please follow the guidelines set by the NPS when visiting.
The posted coordinates will take you to the reconstructed fur trading outpost on Grand Portage Bay, where the information for this Earthcache can easily be found. The hours for this area are 8:30 A.M. to 4:30 P.M. and is only open seasonally, however the answers can easily be found in other parts of the park (such as the Heritage Center and the Grand Portage trail itself) if you are visiting in the off-season or after-hours.
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Grand Portage National Monument is located on Lake Superior's North Shore in extreme northeastern Cook County, Minnesota. It is located about 140 miles northeast of Duluth, and about 35 miles southwest of Thunder Bay, Ontario. It was originally designated a national historical site in 1951, and then as a National Monument in 1958. Covering an area of about 1.1 square miles, it encompasses an 8.5-mile historical portage from a reconstructed British North West Company trading post on Lake Superior to the historical Fort Charlotte on the Pigeon River. Its name in the Native Ojibwe language is Gichi-Onigaming, meaning “Great Carrying Place.”
The Grand Portage had seen use as a critical transportation route for thousands of years. It was part of an ancient transcontinental trade route that connected the Great Lakes to the interior of the American continent. Between 1731 and 1804, thousands of men shuttled tons of supplies and furs over the portage and in and out of warehouses at either end of the woodland trail. Following the British North West Company's withdrawal from the area in 1804, the portage fell out of use. Nowadays the portage trail bisects land within the Grand Portage Indian Reservation and leads from Lake Superior to the present-day international boundary between the U.S. and Canada.
Under the terms of the Webster-Ashburton Treaty of 1842, the use of the trail remains free and open to citizens of both the United States and Canada. This international corridor is of paramount significance to Grand Portage National Monument. Without the Grand Portage, Canadian and American political history and national boundaries very well may have differed greatly.
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The North Shore of Lake Superior contains very old rocks, even for geological standards. Most of the rock in this area is of the Rove Formation, formed about 1.8 billion years ago. About 1.1 billion years ago, diabase dikes intruded into the Rove Formation as a result of the Midcontinent Rift.
At that time, much of northern Minnesota consisted of much taller mountain ranges, and for a time was blanketed with softer rock materials such as sandstone. During the last Ice Age (about 2 million to 10,000 years ago), the bedrock of the area was eroded by a series of advancing ice sheets. The more resistant diabase sills and dikes remained, while the softer shales were bulldozed away by the glaciers. This shaped the landscape of the Grand Portage area that is still seen today in its rugged highland-lowland topography.
Immediately north of present MN-61, the Grand Portage trail circumvents a large ridge by following a small gap in what was formerly a continuous ridge. Over thousands of years, a zone of weakness formed fractures in the hard bedrock of the region. Despite being incredibly erosion-resistant, freezing rainwater combined with tree roots wedged the rocks apart (similar to the process that causes potholes to form on paved roads). This led initially small cracks to eventually widen into the large gap we see today.
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To log this Earthcache, please either message or e-mail me with your answers to these questions:
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What geological feature(s) required the establishment of the Grand Portage? Be as general or specific as you'd like, but don't overthink this.
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Immediately north of MN-61 along the Grand Portage trail, there is a gap in a ridge of hard diabase rock. The process that caused this is described above. Would it be reasonable to say this gap may increase in size in the future? (don't worry, there are technically no wrong answers)
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What is your elevation at the posted coordinates? (anywhere within 250 feet of GZ is fine, due to this area closing in the winter)
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(OPTIONAL, but recommended) - Post a picture of yourself (or an identifiable personal item) either at the posted coordinates or at some other point along your journey.
You can log a find on this Earthcache before hearing back from me, however I may request more detail/clarification later if I think it is required.
All pictures are welcome of your journey, but please do not post pictures that contain answers to the questions - I will have to delete them. In the same vein, please do not post answers to questions in text in your log.
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Thanks to the National Park Service for their understanding and assistance in listing this Earthcache. The help is greatly appreciated.
In addition, I'd like to thank Kurt Franke for indirectly inspiring me to make this with his Earthcache at Minnesota's other National Monument (Pipestone) in the opposite corner of the state.
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The following sources were utilized in the creation of this Earthcache page:
“Grand Portage National Monument (U.S. National PARK SERVICE).” National Parks Service, U.S. Department of the Interior, www.nps.gov/grpo/index.htm.
Schwartz, G.M.. (1950). Geology of Grand Portage and Pigeon Point. Minnesota Department of Natural Resources. Retrieved from the University of Minnesota Digital Conservancy, https://hdl.handle.net/11299/93737.
Thornberry-Ehrlich, T. L. 2019. Grand Portage National Monument: Geologic resources inventory report. Natural Resource Report NPS/NRSS/GRD/NRR—2019/2025. National Park Service, Fort Collins, Colorado.