The ridges above Grand Portage on Lake Superior are held up by dark igneous rock that forced its way into much older sediments about 1.1 billion years ago. Glaciers later scraped the land, and as the last ice left, the area rose out of a series of glacial lakes whose old beaches and bluffs are still there.
A sandstone outcrop at Grand Portage National Monument. From the National Park Service Geologic Resources Inventory.
The monument
| Established | as a national historic site, September 15, 1951; redesignated a national monument, September 2, 1958 |
| Area | 287 ha (710 acres) |
| The portage | 13.7 km (8.5 mi) from Grand Portage Bay on western Lake Superior to Fort Charlotte on the Pigeon River, in northern Minnesota on the border with Ontario, Canada |
| Width | the park owns 30 to 91 m (100 to 300 ft) on each side of the portage trail |
The National Park Service works closely with the Grand Portage Band of Lake Superior Chippewa (Ojibwe).
Two kinds of Precambrian rock
| Rock | Age | What it is |
|---|---|---|
| Rove Formation | about 1.85 billion years | sedimentary rock beneath much of the Grand Portage area |
| Grand Portage dike swarm | about 1.1 billion years (the older generation) | dark diabase dikes, generally trending east, no more than a few tens of feet thick |
| Pigeon River dike swarm | about 1.1 billion years (slightly younger) | far larger dikes that hold up the region's higher ridges, such as Mount Rose and Mount Josephine |
The dikes are linked to igneous activity along the Midcontinent rift, and together they form a roughly right-angled grid of northeast- and northwest-trending bodies.
Ice and water
Successive glaciers eroded the bedrock. Because the dikes and the sedimentary rock resist erosion differently, the ice sharpened that right-angled pattern in the landscape. When the last ice — the Superior lobe — withdrew from the Lake Superior basin about 10,000 years ago, Grand Portage emerged from a succession of lakes in front of the glacier, leaving wave-washed sediments, eroded bluffs, and old beaches and bars.
Part of the geologic map of Grand Portage National Monument. Credit: National Park Service, Geologic Resources Inventory.
Where the detail lives
Grand Portage belongs to the Superior Upland physiographic province and shares its geologic history with a region far beyond the park. The NPS Geodiversity Atlas, drawn from the Geologic Resources Inventory, covers the full variety of a park's rocks, minerals, sediments, fossils, landforms, soils and the processes that shape them. The inventory produces geologic maps and reports for more than 270 natural resource parks, and a Soil Resources Inventory has been completed for Grand Portage; both are in the NPS DataStore.
Sources
Based on "NPS Geodiversity Atlas—Grand Portage National Monument, Minnesota," National Park Service, a work of the United States government in the public domain.
In these publicationsGrand Portage National Monument
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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