Midewin National Tallgrass Prairie
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  • 17 Shaping The Land

    On left, erratic Canadian, granite boulder left by melting glacier 18,000 years ago.

    Over the past 2 million years, four separate mile high glaciers rolled south from Canada down through Illinois as temperatures alternately plummeted and warmed four times throughout this period. As these glaciers moved at a snail’s pace over Illinois, they scrapped the land clean of vegetation, rocks, and soil. While trapped inside the moving ice, these materials were ground to rounded particles ranging in size from clay to boulder resulting in deposits called till. These greedy glaciers held and moved these eroded materials within the frozen ice until warmer temperatures prevailed causing the glacier to eventually melt back north. The last of these glaciers remained here in Illinois until 18,000 years ago before melting completely. This melting caused the glacier to release its load. Since the melting was not uniform, a hilly terrain was created. The melting ice allowed rocks and soil it was carrying to be deposited as moraines. You are now standing on the Rockdale moraine.

    As long as the climate was cold enough to keep the ice frozen, all rocks and soils remained in the glaciers. Eventually, the climate warmed sufficiently for these glaciers to slowly melt back north. A melting glacier deposited the till that once remained in its icy grip. The most recent glacier’s retreat was fairly constant, but sometimes it paused for short periods of time leaving ridges of till called recessional moraines. The Rockdale moraine is an excellent example of a recessional moraine. Many recessional moraines exist from here to central Illinois. This indicates that the glacier experienced many pauses resulting in multiple recessional moraines.

    Along the edge of the Iron Bridge Trail near the Bison pasture lies a small boulder of granite that was left behind from the last glacier. This boulder is called an erratic. A glacier carried it here from Canada. We know the bedrock in this region of Illinois is limestone that was formed when subtropical oceans existed here. Canada’s bedrock is granite which crystalized from a molten state in Canada.  Granite is not a part of this limestone bedrock in Illinois. So, how did that granite boulder get here? Geologists conclude that it had to have been carried here from Canada by a glacier. The sparkly crystals in granite are quartz and mica making it easy to identify. At some point in its melting stage the glacier dropped this erratic along with the other ingredients in till.

    Look to the southeast at the bison pasture. You will see the rolling hills of glacial till called hummocky topography. A glacier left the raw materials of rock and soil during its melting phase. Living things, climate, and topography would induce changes in this raw soil over the centuries until it became extremely fertile. Prairie and farmlands developed on this new rich soil that was now ready for the prosperity of human beings as they migrated into this region setting up homesteads and creating farms for their livelihood.

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