The Ice Age
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| Wisconsin shore St. Croix River Dalles |
The Wisconsin glacial period lasted from about 85,000 - 11,000 years ago. During this time ice covered most of North America north of 38 degrees north latitude to a depth of 2 miles. As the ice sheet melted about 10,000 years ago, the meltwater filled parts of the basins of modern Lakes Superior, Huron and Michigan. The natural drainage of this lake was to the east, but a lobe of the glacier blocked that exit. Water from an area of ice sheet the size of Minnesota and a mile deep filled glacial Lake Duluth until water level was 400-500 feet above the level of modern Lake Superior.
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| Rounded and polished boulders |
When the water overtopped the hills forming a natural dam on the southwest side, the water ran in torrents down the modern Bois Brule and St. Croix River Valleys for years. The water cut easily through Cambrian sandstones, but ran over the top of harder rock. South of here, the rush of water cut deeply into sandstone forming a huge waterfall. The erosive action of the water cut away the foot of the fall and marched upstream and the lip broke loose, eventually cutting into the hard basalt that would form the narrow chute we call the Dalles of the St. Croix River. The force of the water running by here is likened by park interpreters as that of a fire hose. Only the water was full of sand, rocks and boulders. These can be seen in the smoothness of the rocks polished by the action of that sediment-laden water.
But Doctor Hoffman, where did the sand, rocks and boulders come from? Wow, great question, glad you asked. As the Midcontinent Rift formed, water and cycles of freezing and thawing wore down the basalt and the dirt filled the rift valley. In lakes and rivers of the rift valley, sandstones and shales were deposited. Add to that the material that was scooped out by glaciers during the ice ages and deposited at the margins of the glaciers as they melted. Turn on the jets from an overtopped glacial lake dam, and it scours out the loose stuff just like a power washer pushing dirt across your porch or driveway.
Fun Potholes
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| Small pothole cut through rock |
As a torrent of water runs over a rocky bed, there are areas where eddies form. These swirling circular currents will start to scour out a depression in the rock with the sand and rocks that it moves. If a rock rolls into the depression and cannot move out, the water may be able to rock it and roll it around, smoothing the rock into a round grindstone. These grindstones can then swirl around the perimeter of the hole, cutting it deeper. Interstate Park has the highest known concentration of these features and some of the deepest known potholes.
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| Pothole |
The flow of water out of Lake Duluth may have continued for hundreds of years. Only when the Superior lobe of the Laurentide Ice Sheet melted did the water flow downhill toward the east. One can only imagine how much water and force was required to carve out the St. Croix Dalles and these potholes. It must have sounded much like it would standing at the bottom of Niagara Falls. The small pothole figured above is only about six inches across and about two feet deep, tunneling through the entire rock layer. There are over a hundred potholes in the boundaries of the park that are much larger than this. We were actually standing in a twenty feet deep pothole when we took the picture of the left, which shows another pothole that was cut even deeper.
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| Grindstone |
Conglomerates
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| Conglomerate on rocks |
Scoured potholes, grindstones, a gorge cut through tough rock, and rounded boulders all point to a huge flow of water through here. Not just water, but water full of boulders, rocks, pebbles, and sand. How can we be sure? As we were hiking through the woods, we came upon some rocks at least 30 feet above the current level of the water covered with conglomerate. This mixture of sand and pebbles solidified after one of these tremendous floods and cover the rocks in places. The water flowing out of glacial Lake Duluth would have been chock full of this stuff, ideal for cutting through rock. The pebbles in the photograph range up to about 1 inch in width.
Creation of the Park
The basalt in this area was perfect for making gravel for building material, a business venture proposed by a group of St. Paul businessmen in the 1860s. Public interest to preserve the scenery was piqued and influential people like George Hazzard and William H.C. Folsom led the push for preserving the Dalles. George Hazzard was a travel agent, Chamber of Commerce chair and secretary of the Red Rock Camp Meeting Association, which pushed for the combined Minnesota and Wisconsin park idea. Folsom led the legislative push to establish the Minnesota side of the Park. The Minnesota side was established in 1895 and was the second Minnesota State Park, while the Wisconsin side was established in 1900 as the first Wisconsin State Park.
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| Deep pothole |
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| Rock with several small potholes |
Getting There
Waypoint: Latitude 43.3936727 N; Longitude 92.6709847 W
Street Address: US-8/St. Croix Trail and Milltown Rd., Taylors Falls, MN 55084
On I-35 north of the Twin Cities, take Exit 132 to Taylors Falls (US-8 East). The entrance to the Park (Milltown Rd) is in Taylors Falls just west of the bridge to Wisconsin. There is another part of the park in Wisconsin (hence the name Interstate State Park)
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| Pothole filled with debris |
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