Submitted by: Patty Kostro
Although not glaciers, walking along the shores of Lake Michigan in January can sometimes feel like the Ice age!
Imagine that you are standing next to the Willis Tower (Sears Tower) on S. Wacker Drive in Chicago. Look up. Look up as far as you can to the very tip of the antennae (1729 feet). This distance is only a third of the thickness of the glacial ice that covered our region. Our region was covered in glacial ice that was over a mile thick! Imagine the amount of weight that set upon our area from over 5,280 feet of glacial ice.
Glaciers form when snow does not have the opportunity to melt. The continual cycle of new snow falling on old snow causes thickening and eventually this is compressed into glacial ice. Although, glacial ice looks still, it is moving! Glaciers go through phases of advancing and retreating. If snow is added at a faster pace than evaporation and melting (accumulation) a glacier will advance. If evaporation and melting occurs faster than the accumulation (ablation) the glacier will retreat. All of this advancing and retreating is what carves and molds the landscape or topography of an area.
Although glaciers seem to be clean and white from aerial pictures, this is far from their actual appearance beneath or in front of them. Glaciers are filled with rock, sediment, mud and water. The primary reason for glacial movement is gravity. With gravitational forces at play and the weighing down on the land, the meltwaters at the bottom force the glacier to move. As the glacier moves it picks, plucks and erodes the bedrock beneath it.
There are two types of glaciers: Alpine and Continental. Alpine glaciers begin at the top of a mountain and flow downward in the same fashion as a river. Continental glaciers are much larger than alpine glaciers and are massive sheets of ice that flow outwards in all directions. Lake Michigan and the Great Lakes were formed due to a massive continental glacier called the Laurentide Ice Sheet. This ice sheet covered over 5 million square miles of land and in some areas it had a thickness of up to three miles. It was lying on nearly all of Canada and reached its furthest southern points in the Midwest of the United States.
The Laurentide Ice Sheet is outlined in blue as it appeared 15,000 ago. ( Map courtesy of www.ncdc.noaa.gov)
The Great Lakes were made due to a series of lobes that extended out from the glacier. Lobes are tongue or finger like projections that appear as the glacier is retreating. There were several lobes that covered the Great Lakes area. The major lobes are named after the Great Lake that it created. Thus, the lobe that is responsible for the creation of Lake Michigan is respectively named the (Lake) Michigan ice lobe. Prior to this lobe’s final retreat around 14,000 years ago, our area had gone through several thousands of years of the glacier advancing and retreating.
The image shows the lobes of the last glacial maximum of the Laurentide Ice Sheet approximately 14, 500 years ago. (Image courtesy of Lawson et al., 2011)
The advancing and retreating creates depositional landforms called moraines. Moraines are made of the rock debris (till) that has been scraped up by the glacier as it is advancing, but then all of the debris is left behind as it is retreating. The best way to imagine this is to think of a bulldozer that comes along and carves out the land and then just dumps it. So, in our area we can have pieces of rock (till) that originated as far north as Canada. The only way this type of rock could geologically be here is by being carried by glacial ice.
We can find evidence of the movement of the glacier by the moraines that have been left behind by the glacier. But, it is the moraines that allowed Lake Michigan to be created as the ice was retreating. The moraines actually act like a dam and were able to hold the water that was being melted. Because of the damming and retreating of ice, along with areas within the northern boundaries of Lake Michigan that were opening and closing (like plugs to a drain), Lake Michigan went through several different sizes, shapes and water elevations before it reached the appearance that it has today.
So, where are the dunes in all of this? The beginning of our dunes started with the glaciers. Without the creation of Lake Michigan, we would not have dunes. This will be the topic of next month’s blog. The appearance of Lake Michigan from glaciation until present and how the changing water elevations helped in shaping our shoreline as we know it today.
Places to see: Flint Lake and Loomis Lake north of Valparaiso are lakes within the Valparaiso Moraine and are both examples of kettle lakes. Kettle lakes are formed when very large chunks of ice from a glacier break off and are partially buried. The ice eventually melts and forms a lake in a deep depression that resembles a kettle.
Flint and Loomis Lake map
Read more about the Geology of the Dunes right here!