An editor at a magazine once asked me to do a calendar on what there is to appreciate in nature each month in the midwest. I started in May, when the issue was coming out, talking about migrating warblers and wild asparagus. I clicked along through each month–sunflowers for August, sandhill cranes for November–until I hit a wall with February. I chewed my pen tip, looked through field guides, and tried to think of something good to say. Finally I wrote that February was a good time for a trip to Costa Rica.
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Since then I’ve learned that there’s more to nature in Chicago than plants and animals. There is something worth watching in February: potholes. They’re plentiful and easy to spot. And when you find one it can provide an interesting lesson in geology. Most geological formations in northeastern Illinois–the ancient beach ridges, the rivers, Lake Michigan–were formed 10,000 years ago when the glaciers receded. And most geological changes under way right now won’t be noticeable for thousands of years. In contrast to aeons-long and hard-to-envision geological occurrences, potholes provide instant gratification. Roads aren’t real rock formations, but that doesn’t matter–nature doesn’t discriminate between rocks placed by God and those laid down by the Illinois Department of Transportation. The same forces at work on a natural sandstone outcrop also operate on the Dan Ryan Expressway.
In a process Wiggers calls “frost wedging,” rain collects inside a crack in a road surface. When the temperature drops, the water turns into ice. Ice, a solid, takes up more room than water, so it squeezes against the concrete. This process goes on for years, through many seasons of freezing and thawing.
