Hennepin County

Where Found: 

Located off West River Road on the bank of the Mississippi River near the Coon Rapids Dam

Text On Markers: 




About 20,000 years ago, a glacier from the Ontario region passed through the Lake Superior basin and reached the Twin Cities area. As the glacial ice melted, it deposited the St. Croix moraine at its margin. A moraine is a deposit of sediment (clay, silt, sand, gravel, and boulders) left by a melting glacier. The St. Croix moraine forms a belt of hills that crosses the Twin Cities and extends northwest to St. Cloud. As the moraine was deposited, it buried some blocks of stagnant glacial ice that were left stranded in an ancient valley.

This park lies within the southern part of the Anoka sand plain, a vast area of sand stretching from St. Cloud to the St. Croix River valley. Much of the sand was deposited about 12,000 years ago in Glacial Lake Anoka. Fed by meltwater from glacial ice in northwestern Minnesota, the lake formed on the northern side of the St. Croix moraine. The hidden ice blocks buried earlier in the moraine eventually melted to create a gap through which Glacial Lake Anoka drained, abandoning its former outlet to the east through the St. Croix River valley. With this gap, the general course of the Mississippi River was established between the sites of north Minneapolis and Hastings.

After Glacial Lake Anoka drained, the Mississippi River meandered across the flat surface of the former lake bottom. The shifting river formed two distinct terrace levels before becoming entrenched in its present narrow floodplain. The terraces can be seen about two kilometers west of here on 109th Avenue, where a northwest-trending slope separates Champlin Park High School on the upper terrace from Jackson Middle School on the lower terrace. As the Mississippi cut below the lower terrace, it encountered many boulders in the glacial sediment just upstream from this dam. The boulders formed a barrier to erosion on the river bottom, creating the Coon Creek rapids, now submerged behind the dam.

Downstream from the dam, the river has cut its channel into reddish, clay-rich sediment from a glacial lake that existed before Glacial Lake Anoka. The red clay was mined along Coon Creek just north of this park by various companies from just before 1880 to about 1910.

Erected by the Geological Society of Minnesota in partnership with the Minnesota Department of Transportation, the Minnesota Geological Survey, and the Three Rivers Park District. 2003

Marker Number: 



Metro Region

Marker Location: 

GEOLOGY OF THE COON RAPIDS DAM AREA Minneapolis 45° 8' 31.9272" N, 93° 18' 47.1852" W

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Do it yourself Region: 

Metro Area


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