Sleeping Bear Dunes Geology

The shoreline, the hills, the valleys, the small lakes, and the sand dunes you see today are evidence that powerful earth-moving forces of ice, wind water have been at work here. Often geological changes occur slowly over millions of years, but here you can witness dramatic changes over your lifetime. Twice in this century sandslides at Sleeping Bear Point sent large land masses plunging into Lake Michigan and in summer of 1998, a slide took thousands of tons from Pyramid Point.

During the Ice Age continental glaciers spread southward from Canada, repeatedly burying this area under sheets of ice. These massive glaciers enlarged river valleys, carrying out the wide, deep basins of the Great Lakes. They deposited huge piles of sand and rock debris when they melted, leaving behind the hilly terrain you see today. Finally, 11,800 years ago, the last glacier retreated.

With the glacial landscape formed. Lake Michigan and many smaller lakes began taking shape. The level of water filling Lake Michigan’s ice carved basin rose and fell many times before reaching its present level. The lake’s shoreline — at first irregular with jutting headlands and recessed bays — was gradually smoothed out. Waves wore back the headlands. Shoreline currents carrying sediments build sandbars and spits across bay mouths. Sometimes sediments dammed bays, creating small inland lakes such as Glen Lake near the Lake Michigan shoreline.

The glaciers left behind an ideal setting for building sand dunes: a sandy coast on the leeward side of Lake Michigan. Prevailing westerly winds blowing across the lake build two kinds of sand dunes in Sleeping Bear Dunes. Beach dunes develop on low-lying shores of Lake Michigan. Their main ingredient is beach sand. The Aral Dunes, along Platte Bay’s north shore, are good examples of beach dunes. Perched dunes, on the other hand, sit high above the shore on plateaus. Glacial sands atop these surfaces supplied material for these dunes. The Sleeping Bear Dune of Indian legend is a perched dune as are the dunes at the south end of South Manitou Island.

Some dunes migrate, pushed by the wind. Sometimes shifting sands bury trees. Then, as the dunes move on, ghost forests of dead trees are exposed, stark reminders of the dunes’ passing. Not even man has escaped the influence of windblown sand. U.S. Coast Guard buildings now located in Glen Haven had to be moved from Sleeping Bear point in 1931 because migrating dunes threatened to cover them.

Beach grass and sand cherry are among the first plants to grow on newly built dunes. They play an important role in dune development. They help build dunes by acting as obstacles that slow sand-laden wind and force it to drop its load. Their roots hold sand in place and stabilize the dunes. But if a strong wind succeeds in stripping the plants from a dune, a bowl-shaped blowout can be excavated in the exposed area. Vehicles are prohibited on the dunes because they destroy dune vegetation. Tire track scars last many years.

Top photo: Rising Sun wide by anttler

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