The Story of the Sand Dunes

 

 

The Sleeping Bear Dunes Complex covers an area of approximately four square miles and is bounded by a wall of sand up to about 110 feet (34 meters) high. People often wonder, “Where did all the sand come from?” To answer that question we must consider a remarkable time in the earth’s history: The Ice Age.

 

The Ice Age

Beginning about ten million years ago, the earth’s climate began to cool, leading to the great Ice Age of the last two million years. During the Ice Age, continental glaciers alternately advanced and retreated. At its maximum extent, the ice reached as far south as southern Illinois, Indiana and Ohio. Only about 11,800 years ago the last glacier retreated from this part of Michigan.

The cause of the Ice Age is still debated among scientists, but the formation of glaciers is clear. All it takes for a glacier to develop is for more snow to fall in the winter than melts in the summer. When this continues over a period of years, an ever-deepening, permanent snow cover develops. Through melting, refreezing and the weight of the overlying snow, the lower layers gradually turn to ice. When the ice becomes thick enough, it begins to flow. At this point it becomes a glacier.

As a glacier moves across the land, it picks up rocks, gravel and other debris which become frozen into the lower layers of the glacier. As a result, the glacier becomes a powerful agent of erosion. The Great Lakes’ basins were once river valleys that were widened and deepened by glaciers' erosional properties.

Examples of glacial action are seen in the hilly moraines of this area. The glacier acted as a conveyor belt, carrying rock debris to new locations. When the rate of forward motion was balanced by the rate of melting, the glacier paused long enough to leave a ridge of glacial debris. The resulting ridge is called a moraine. Two types of moraines found in the park are both headland and highland moraines.The park’s shoreline with its pattern of headland and low-lying embayments indicates the shape of the ice during the final melting.

 

Changing shorelines

Shoreline erosion and deposition are important in making sand available for dune development. Waves undercut the headland bluffs and cause sediments to fall into the lake. Longshore currents carry the sediment to bays where particles settle in the quieter water. Wave action can sort sediment by size; the coarser sediment, including sand, settles near the shore, while the finer particles are carried in suspension to deeper water. In this way sand is concentrated along the shoreline and is available for building beach dunes. As headlands are worn back and bays fill in, the shoreline takes on a straighter alignment.

 

Waves wore back the headlands and currents carried sediments to bays, building new land. As a result, the irregular shoreline became straighter.

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The Sand Dunes

A sand dune is simply a pile of sand deposited by the wind. The word “sand” refers to a particle size- approximately the size of a grain of sugar. Sand happens to be just the right size to be blown by the wind and then deposited to build a sand dune. Often a dune forms around an obstacle such as a rock or clump of grass where the wind begins to lose speed. This small dune itself becomes an obstacle and causes more sand to be deposited. As a result, the dune grows.

 

Dune grasses creating an obstacle to moving sand.

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Dune Classification

Here at Sleeping Bear Dunes National Lakeshore, dunes can be divided into four categories--beach dunes, perched dunes, falling dunes and “de-perched” dunes--based on their location and mode of origin.

Beach dunes, common along Michigan’s western coast, form near lake level as onshore winds carry beach sand inland. Beach dunes occasionally grow to considerable height: the Aral Dunes along Platte Bay, for example rise over 100 feet (30 meters) above Lake Michigan. Perched dunes rest on a headland, often several hundred feet above lake level.

Perched dunes occur on the Sleeping Bear Plateau, Empire Bluffs, Pyramid Point and the west coast of South Manitou Island. The perched dunes of the Sleeping Bear Plateau are actually a relatively thin blanket of wind-blown sand resting on a thick deposit of sandy glacial debris. When the wind reworks the upper layers of glacial sediment, sand is deposited into dunes while the coarser material remains behind as a lag gravel. Silt and clay-sized particles are so light that they remain in suspension and travel a long distance before settling. The wind is an effective sorting agent and dune sand shows little variation in size.

Falling dunes evolve from perched dunes when sand migrates off the plateau and onto an adjacent lowland, as is happening at the Dune Climb. Ultimately, the dunes may move beyond the plateau and rest solely on the lowland area. Such dunes may be considered “de-perched”, which is an informal term describing the situation over part of the Sleeping Bear Point lowlands.

 

Dune Movement

Active dunes gradually advance over time. In some places trees and telephone poles have been buried indrifting sand over a period of several years. A common question is “How fast are the dunes moving?” Rates of sand movement vary from one place to another and from one year to the next. At the Dune Climb the average rate of advance has been about 4 feet (1.2 meters) per year over the past few years.

Direction of sand movement also varies from one place to the next. The shapes of dunes reveal the direction of the winds. Dry, loose sand can take on a maximum slope of 34 degrees, known as the angle of repose. As sand piles up on an unvegetated dune, the leeward (downwind) slope approximates the angle of repose, while the windward slope is more gradual. Ripples in the sand show a similar asymmetrical shape.

Dune sand moves primarily by a process called “saltation”, literally “jumping”. Wind forces grains of sand to roll, collide with other grains of sand, and in subsequent collisions some grains bounce into the air, are driven downwind as they fall, and upon landing strike other grains to produce a chain reaction. On a windy day you can see a hazy zone of sand moving just a foot or two above the ground.

The most abundant mineral in the local sand is quartz, which is notable for its resistance to physical and chemical breakdown. Some grains are colorless while others have an iron oxide coating and appear rusty. Other minerals of the sand include pink or white grains of feldspar, black magnetite and hornblende, red garnet and green epidote.

Because of its hardness, quartz sand makes an effective tool of erosion. On the Sleeping Bear Plateau, windblown sand has produced ventifacts, rocks with one or more smooth flat surfaces like the facets of a gem. While the movement of sand produces impressive changes, ultimately that movement may be limited by several factors, a major one being the stabilization of dunes by plant cover

 

Plants of the Dunes

Beachgrass, sand cherry and cottonwood are among the first plants to appear on the dunes. As plant decay enriches the soil, conditions become suitable for other plants and eventually a forest may develop in the embayment dune area. However, the plant community may be disturbed occasionally by wind erosion or drifting sand.

Wind erosion of a plant-covered dune produces a blowout. The intense erosion of the windward side results in a bowl-shaped dune with stabilized arms pointing windward and sand overflowing on the leeward slope. The Sleeping Bear Dune illustrates this shape, but it was not always this way. Old photographs of the dune show a densely vegetated mound, vaguely resembling a sleeping bear. As wind erosion destroyed the original shape and much of the vegetation, the interior of the dune was revealed. Buried soil layers and a “ghost forest”--the skeletal remains of trees swallowed up long ago by drifting sand--tell of past intervals of stability and change.