The Geology P.A.G.E.: Geology of the National Parks in Pictures

Thursday, November 05, 2020

Geology of the National Parks in Pictures - Sleeping Bear Dunes National Lakeshore

My next post about the Geology of the National Parks Through Pictures is a park we visited back in the summer of 2005.

You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.

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Sleeping Bear Dunes National Lakeshore

On the northeastern shore of Lake Michigan, lies these monstrous dunes that run right up to the edge of the lake. Like Indiana Dunes National Park to the south, this is a glacial landscape that eventually was carved up by the lake.

Here is a view out over Lake Michigan from the top of the dunes. When the glaciers had come through this area they had carved out the river valleys, increasing the depths and widths of the valleys in this area so much that it created the Great Lakes. The glaciers acted as giant plows, scooping up whatever was in their path and carving out the ground surface over which they rode. Towards the end of the glacial ice sheets, they acted like conveyor belts. The materials that were scooped up along the way were then deposited in piles (called moraines) of glacial debris (called till) where the glaciers melted. As the glaciers retreated (melted away) they deposited moraines along the path of their retreat.

The top plateau of Sleeping Bear Dunes is formed by a moraine from the retreating glaciers. After the initial retreat of the glaciers, two small glacial advances were made. The first, called the Port Huron, covered the area with glaciers, further reshaping the landscape, including the previously deposited moraines. The final glacial advance ~11,800 years ago, termed the Greatlakean, terminated at the Lake Michigan shore. Sandy till deposited by this glacial advance, as well as glacial outwash sediment that contained abundant amounts of sand, were the primary sources for sand within the region, especially upon the moraine plateau.

After the retreat of the glacier, wind and wave action helped to reform the shoreline smoothing out headlands and filling in the nooks and crannies. The lake level also fluctuated significantly during high and low periods until finally settling down to its elevation of 577 feet above sea level. Sleeping Bear Dunes were deposited when the lake level was still fairly high, at 612 feet above sea level.

Along the shoreline there are several steep drop-offs that are on average 200 to 300 feet high. Erosion by wave action over the last several thousand years had helped to emphasize these dramatic cliffs that cut into the moraine situated below the sand deposits. Wind also has an effect on the still active dunes atop of the plateau, called "perched" dunes. Wind not only moves these dunes around on top of the plateau but is capable of blowing minor amounts of new of sand up the cliff faces to add to the dunes.

Not all of the shoreline is that steep though. Here you can see a dune with the stabilizing beach grass growing fairly well along it. There are dunes along the shore as well, located below the plateau at lake height. These dunes are regularly fed with new sand from the westerly winds that blow across Lake Michigan.