Glacial Erratic

Lockport Prairie Nature Preserve lies within a glacial drainageway that once carried the waters of Lake Chicago, a predecessor to today’s Lake Michigan, southwestward through a low spot in the Valparaiso Moraine. The flow ended several thousand years ago when a slightly lower passage for glacial meltwater opened eastward down the St. Lawrence River, but the ancient flow left behind steep bluffs, exposed dolomite bedrock, gravel bars, and the occasional boulder. The boulder pictured here is likely a “glacial erratic,” a rock of distant origin deposited in this area by a glacier and further relocated by the torrent of meltwater. The torrent also left behind a convenient route for early travelers to cross from the Great Lakes to the Mississippi River drainage on the lesser flow of the Des Plaines and Illinois Rivers that now occupy the Lake Chicago outlet valley. This path was later capitalized on by engineers looking for ways to move both goods and waste water to the Mississippi from Chicago.

Today this portion of the Chicago Metropolitan Area is a heavily used urban and industrial corridor but the Lockport Prairie Nature Preserve, managed by Will County Forest Preserves, protects the unique species of this rare dolomite prairie and provides a chance for any interested observer to see a small piece of the Lake Chicago outlet valley as it has existed for many centuries.

In this view, the late June, modern-day condition of the prairie occupies the lower portion of the canvas, while I chose to remove the 9th Street bridge, industrial structures, and numerous trees from the rest of the view to show the valley in its pre-European settlement condition. The Des Plaines River can be glimpsed near the distant tree line. Because the river served historically as a barrier to prairie fires blowing in from the west, groves and woodlands predominate on the protected islands and east bank while open prairie predominates on the west side. No doubt, the blooms of black-eyed Susans, smooth phlox, and prairie Indian plantain would also have been on display long ago.