Sleeping Bear Dunes Excerpt from Great Lakes Rocks

by Danielle Coty on July 8, 2019

Image of Sleeping Bear Dunes National Lakeshore to accompany book excerpt from Great Lakes Rocks

Every year over 1,000,000 people visit the Sleeping Bear Dunes National Lakeshore, lauded by ABC’s Good Morning America as the “Most Beautiful Place in America.” Whether they are visiting to climb the massive sand dunes, to stargaze, or for the scenic drive, visitors have the opportunity to marvel at a scenic lakeshore where sand dunes seem to rise straight out of Lake Michigan.

Yet the formation of the Great Lakes dunes is only one of the ways in which geology manifests in the region. With natural resources such as salt and natural gas, formations such as the Pictured Rocks National Lakeshore, and the Great Lakes themselves, the economy and beauty of the region are both tied to its geology. For those wondering how such wonderful natural features in the Great Lakes area came to be, they can learn more in Stephen Kesler’s recent book Great Lakes Rocks: 4 Billion Years of Geologic History in the Great Lakes Region.

Excerpt from Chapter 2: Landscaping the Continent

2.5. Coastal Dunes Are Major Features around Lakes Michigan and Huron

Sand dunes are widespread in the Great Lakes region. Most of the dunes are at or near the present shoreline of the Great Lakes, although there are also some inland dune fields (fig. 2.6). The largest dunes are found along the south and east side of Lake Michigan and the Lake Huron shore. Several important national, state, and local parks are centered on dunes, including the Sleeping Bear and Indiana Dunes National Lakeshores along Lake Michigan, Sandbanks Provincial Park on Lake Ontario, Inverhuron and Pinery Provincial Parks on Lake Huron, and the Grand Sable dunes in the Pictured Rocks National Lakeshore on Lake Superior.

There are two types of Great Lakes dunes. The simpler type, foredunes, form ridges up to 5 m high paralleling the shore and on the lake side of the beach. These form during low stands of the lake that allow wind to sweep sand shoreward. Because they are directly on the beach, these dunes are very easily removed or modified by storms. The more complex type of dune, known as transgressive dunes, are much larger, reaching heights of 60 m, and are found farther onshore from the beach. Some transgressive dunes are actively moving, although most have been stabilized by vegetation (fig. 2.9A in color section). Some dunes reach additional elevations because they formed on top of sandy glacial deposits, where prevailing winds picked up sand to make what are called perched dunes. Both the Sleeping Bear and Grand Sable dunes in Michigan (fig. 2.9B in color section) are of this type. Some transgressive dunes, such as those south of Marquette, Michigan, are not found along coasts and appear to have formed when wind concentrated sand on plains of outwash that formed in front of the retreating glaciers.

These dunes are the product of glaciers, lakes, and wind. The glaciers ground the rock into sand grains, wave action along glacial and modern lakeshores concentrated the sand, and the wind piled it up into dunes. This process probably continued intermittently throughout the more than 2-million-year-long Pleistocene glacial epoch, which is discussed in chapter 3, although the dunes we see today formed during and after the last glacial retreat starting about 10,000 years ago, and they have provided an interesting window into the most recent postglacial history of the region. In a continuing program of study, Alan Arbogast and his associates have used isotopic and other methods to measure the age of sand in the dunes, as well as the age of ancient soils (paleosols) buried beneath the dunes. Using this information, they have shown that the first stage of dune formation coincided with the Nipissing high stand of the Great Lakes about 5000 years ago, which is discussed in the next chapter. At this time, Lakes Superior, Huron, and Michigan were at the same level, about 10 to 15 feet above the present level. Another phase of dune formation took place between about 3500 and 2000 years ago, and this was followed by a final phase of dune growth between about 1000 and 500 years ago.

The latest of these dune growth phases coincided with the Medieval Warm Period,15 when Great Lakes climates were drier, and a growing body of work indicates a more regional control on the dunes. For instance, Walter Loope and others reported that dune growth happened when a drought that affected the entire Great Lakes Basin caused a loss of forest cover. Dune sand can also be distinguished from beach sand, and concentrations of these two types of sand in deposits in small lakes along the Lake Michigan shore-line have been used to determine the history of lake levels and periods of high wind activity. In a recent study, Timothy Fisher and others showed that periods of high dune sand in the lakes could be correlated with solar cycles, suggesting that dune mobility is controlled by climate. Because dunes change so rapidly, they are relatively unstable, and many dunes along shorelines are subject to landslides (box 2.3).

 

Read More

You can read more from Great Lakes Rocks in Stephen Kesler’s recent guest blog post.

Comments on this entry are closed.

Previous post:

Next post: