Dunes are largest freshwater system in the world
Published 11:55 am Friday, June 4, 2010
By TERRI GORDON
Off the Water
The system of dunes extending from Indiana to Mackinaw are the largest freshwater dunes in the world. Formed in three separate eras, they can rise to 200 feet above the lake. Wooded or open, they support a wide variety of habitats and plant species.
Michigan’s sand dunes started with the last ice age, and a glacier that formed just west of Hudson Bay and spread in all directions, joining other “lobes,” and extending to the Ohio River. According to Chuck Nelson, naturalist and director of Sarett Nature Center in Benton Township, these glaciers retreated in “jerks and stages,” creating moraines. “They would stop for a thousand years and just sit there,” he said.
Glacial activity is responsible for two of the three things needed for dune formation. It provided deposits that became sand, and it created the platform upon which the dunes sit.
Nelson refers to this as the “stage.” The only other necessity is a force to pile the sand onto the stage. In Michigan, that force is wind. Winds come from the west – and with nothing to block them as they come across the lake, they are unbeatable conveyers of sand.
Ten thousand years ago, the glacier covering the area was melting. ‘It was still moving south,” Nelson explains, “but it was melting faster than it was travelling, so it was ‘backing up,’ and seemed to be moving north. The end result was that it made a moraine, a big hill, a glacial moraine, and it piled it up right about where the shoreline of Lake Michigan is right now.”
Locally, the biggest part of this moraine is the Covert Ridge. Made of rock, or clay, the moraine prohibits dune formation.
“Red Arrow (Highway) goes along this ridge (between Stevensville and St. Joseph) and you can’t form sand dunes,” Nelson said. “But low and behold, this Covert Ridge leaves the lake edge (coming south from St. Joseph) right about where Cracker Barrel is, and it goes inland.” The flat, open space left between the Covert Ridge and Lake Michigan extends south to Lakeside and is called the Grand Moraine Embayment. It is the perfect landscape for dune formation.
And so, with sand, wind and a platform, a sand ridge started forming. At first, the dunes ran parallel to the shore, some of them stabilizing into forested rises, but where breaks occurred, wind pushed sand back into “blowouts,” creating u-shaped, or parabolic dunes.
“And pretty soon the whole ridge was made up of parabolic dunes,” Nelson said. “There were literally hundreds of them.”
For about three thousand years, a period known as the Algonquin Era, dunes formed. Then, during the Chippewa Era, about seven thousand years ago, Lake Michigan all but disappeared.
“It drained to a little puddle in the middle of the lake,” Nelson said. “What happened is this: The ice went farther and farther north until it got farther north than the St. Lawrence River, and the whole lake, when it got to there, drained out through the St. Lawrence River and through the next lake and the next one and right over the Niagara Falls.
“But the ice kept on going, and it finally got far enough away that the spring in the earth’s crust, suddenly came back up – isostatic rebound, they call it – and after the release of the weight of the ice, up it came, to about the present level, dammed up all of the drainage, and here comes the lake, forming up high again.” Another line of dunes formed, just like the first, and again, breaks allowed the wind to push the sand back into parabolic dunes, “right over, or in-between the other dunes,” Nelson said. These are called the Nipissing Dunes, and are the biggest, says Nelson, because the Chippewa period left a large amount of sand.
At this point, the lake found an old crack in the earth that allowed it to drain to the south, and its level dropped again, until about two or three thousand years ago, when an earthquake “shut the drainage off,” letting the lake rise again.
“So now we have a third epic going on,” Nelson said, “and sure enough it formed another nice line of dunes. And that’s what you’re looking at today. You are looking at the Algoma Epic.”
Sadly, many dunes have been lost as humans have levelled them to build cities, or mined them for industrial use. Given the processes that formed them, it is good that many have also been preserved as beaches for people to enjoy.