Near Grand Marais in Cook County, Minnesota — The American Midwest (Upper Plains)
Geology of Good Harbor Bay
July 25, 2022
1. Geology of Good Harbor Bay Marker
Inscription.
Geology of Good Harbor Bay. . The rocks of the North Shore of Lake Superior record the last period of volcanic activity in Minnesota. This volcanism occurred 1.1 billion years ago when the North American continent began to rupture along a great rift valley, which extended from the Lake Superior region southwest to Kansas. As this rift valley opened, basaltic lavas erupted into it intermittently for about 20 million years, accumulating to a thickness of up to 20 kilometers in the Lake Superior region., With each eruption, red-hot lavas fountained from kilometer-long fissures for up to decades at a time, flooding over large areas of a barren landscape. Flood basalt eruptions typically followed one another in geologically rapid succession, but at times there were significant intervals (thousands to millions of years) without volcanic activity. During such intervals, streams and rivers flowing over and eroding the volcanic terrain would deposit sediments into lakes in low-lying areas. When volcanic activity resumed, these sediments could in turn be buried, heated, and compacted by lava flows and transformed into sedimentary rocks., An example of such a geological cycle of eruption, sedimentation, and renewed volcanism appears in the cliff face across the highway from this marker. Beneath a dark-gray basalt flow is a reddish, thinly bedded siltstone, sandstone, and shale formation. Beneath these sedimentary rocks is another lava flow, which is exposed in the creek bed of Cut Face Creek just north and down the hill from this road cut. The full thickness of this sedimentary rock formation is about 40 meters. This thickness indicates a prolonged lull in volcanic activity, perhaps lasting several million years. The broken-up and mineralized character of the basalt at the left side of the cliff face resembles features observed when lavas explosively encounter standing water. This and the fine sediments beneath the lava suggest that a shallow lake may have existed in the area at the time of renewed volcanism.
The rocks of the North Shore of Lake Superior record the last period of volcanic activity in Minnesota. This volcanism occurred 1.1 billion years ago when the North American continent began to rupture along a great rift valley, which extended from the Lake Superior region southwest to Kansas. As this rift valley opened, basaltic lavas erupted into it intermittently for about 20 million years, accumulating to a thickness of up to 20 kilometers in the Lake Superior region.
With each eruption, red-hot lavas fountained from kilometer-long fissures for up to decades at a time, flooding over large areas of a barren landscape. Flood basalt eruptions typically followed one another in geologically rapid succession, but at times there were significant intervals (thousands to millions of years) without volcanic activity. During such intervals, streams and rivers flowing over and eroding the volcanic terrain would deposit sediments into lakes in low-lying areas. When volcanic activity resumed, these sediments could in turn be buried, heated, and compacted by lava flows and transformed into sedimentary rocks.
An example of such
Click or scan to see this page online
a geological cycle of eruption, sedimentation, and renewed volcanism appears in the cliff face across the highway from this marker. Beneath a dark-gray basalt flow is a reddish, thinly bedded siltstone, sandstone, and shale formation. Beneath these sedimentary rocks is another lava flow, which is exposed in the creek bed of Cut Face Creek just north and down the hill from this road cut. The full thickness of this sedimentary rock formation is about 40 meters. This thickness indicates a prolonged lull in volcanic activity, perhaps lasting several million years. The broken-up and mineralized character of the basalt at the left side of the cliff face resembles features observed when lavas explosively encounter standing water. This and the fine sediments beneath the lava suggest that a shallow lake may have existed in the area at the time of renewed volcanism.
Erected 1998 by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation and the Minnesota Geological Survey.
Location. 47° 43.574′ N, 90° 26.549′ W. Marker is near Grand Marais, Minnesota, in Cook County. It is on North Shore Scenic Drive (State Highway 61) 2.3 miles east of County Road 7, on the right when traveling north. Located at the Good Harbor Bay Overlook on the west side of Good Harbor Bay. Touch for map. Marker is in this post office area: Grand Marais MN 55604, United States of America. Touch for directions.
Regionally, this marker is on Minnesota’s Boundary Waters Canoe Area Wilderness, in the Arrowhead Region, in the Iron Range, and in the North Shore. It is also in the American Midwest, on the Great Lakes, and in the Corn Belt. Globally, it is in North America, in the Great North Woods, on Lake Superiors North Shore, in the Western Hemisphere, in the Western World, and in the Anglosphere. Historically, it finds itself in what was once the Viceroyalty of New France, Ruperts Land, the territory of the Mississippian Culture, and the Northwest Territory.
Other nearby markers. At
July 25, 2022
3. Geology of Good Harbor Bay Marker, from the northwest
4. The cliff face across the highway from the marker
July 25, 2022
5. Neighboring S. Rex Green Marker
In Honor of
S. Rex Green
A Man of Foresight
As engineer of lands and right of way, he helped establish this highway for the public's full enjoyment of the glories of forest, stream, and lakes
Credits. This page was last revised on August 3, 2022. It was originally submitted on August 2, 2022. This page has been viewed 923 times since then and 88 times this year. Photos:1, 2, 3, 4, 5. submitted on August 2, 2022.
