Glaciers / Glacial Features
As you enjoy your visit through Sleeping Bear Dunes National Lakeshore and northern Michigan, think about ice. Think about ice three thousand feet higher than the moraine hills around Glen Lake, and in some places, more than a mile thick. This ice sheet covered more than half of the North American Continent including Minnesota, Wisconsin, Michigan and down into Iowa, Illinois, Indiana, and Ohio. Imagine ice so thick and heavy that it actually pressed down the Earth’s crust? It moved, it bull dozed, it scooped up huge Precambrian boulders and other rocks from up north and carried them to destinations here. Imagine the power of the forces and the length of time it took to form the hills and valleys, shorelines, lakes, and streams of today.
The last advances of the glaciers, the Port Huron and Valders substages of the Wisconsin ice sheet, followed valleys carved out by the earlier, larger glaciers. The last of the glacial ice melted away from this area 10,000 – 14,000 years ago. The headlands and moraines from earlier, larger glacial ice lobes, such as Pyramid Point, Sleeping Bear Plateau, and Empire Bluffs steered more recent glacial ice into Good Harbor and Shell Lake, School Lake, Lime Lake, Little Traverse Lake; Sleeping Bear Bay and Glen Lake; Platte Bay and North and South Bar Lakes and Platte Lake. Occasionally, huge blocks of ice like icebergs would break off from the receding glaciers and became surrounded and covered by gravel, sand and debris from the glacier. As these ice blocks melted, a great conical depression would form as the material covering the ice block collapsed. These huge depressions are called kettles. There are many of these throughout Michigan and several are in the park.
As the glaciers melted, the Great Lakes basins filled with water. As the ice rapidly melted from the ice lobe in Sleeping Bear Bay it could not flow north because of the remaining ice. A river carried this water south from present day Glen Lake, along where M-22 is now, around Empire and finally into the Lake Michigan Basin by Otter Creek. A look at this glacial drainage channel (now dry of course) as it crosses M-22 at Stormer Road, provides a visual of the immense volume of melt water it once carried.
Wave action over thousands of years, from Lake Michigan and other earlier, greater lakes that filled the basin, truncated the moraine headlands so that they now appear as great headland bluffs. This formed the steep sand and gravel faces so prominent at Empire bluffs, the Sleeping Bear bluffs and Pyramid Point. This process continues, especially during years of high water levels in Lake Michigan, so that great masses of these bluffs occasionally collapse into the lake.
North Bar and South Bar Lakes near Empire are embayment lakes and on a grander scale, so are Glen Lake and Crystal Lake. The lakes were formerly embayments of the higher Lake Alqonquin, which preceded Lake Michigan. They were open to the larger lake, but were cut off by sand moving steadily along the shoreline as the lake receded. This action can be seen today along the Lake Michigan shoreline. The prevailing winds are from the southwest and generate waves that move north along the shoreline. Wave after wave, passes a child’s sand shovel-full size of sand to the north and the next wave does the same and the next the same; passing it on. Similarly, storm waves move big shovel-full amounts of sand to the north; wave after wave.
Some of the most prominent features in the park are the perched dunes. These are not ordinary beach dunes. They are high dunes perched on top of already high glacial moraines. The moraines are the great ridges of material that the glaciers pushed before them and then deposited when they halted and began to recede. As waves and wind along the lakeshore cut away the headlands of the glaciers, the wind blew the lighter sands higher up on top of the moraines. The heavier stones fell down toward the beach. This continual sifting process can be experienced on a windy day from the Lake Michigan overlook on Stocking Scenic Drive. You will see and feel the sand blowing upward and you may carry some of it away in your eyes, ears and hair.
All of these awesome geologic forces happened over thousands of years. The Pleistocene Epoch, considered the Ice Age, may have lasted, according to geologists, 500,000 to 2,000,000 million years. The last great glacier that covered all of this land, the Wisconsin Ice Sheet, occurred 50 to 70 thousand years ago. This was preceded by earlier ice sheets during the Illinoisan, the Kansan, and the Nebraskan ice ages. Each advanced and retreated over many thousands of years.
More information on the geology of the Sleeping Bear Dunes area is available at the National Park Service Geology Fieldnotes web site.