Nature Notes

Thinking About Cold on a Warm Day

By Boak Wiesner

Even though I’ve been traipsing around the Arboretum pretty often over the last five years, am I nevertheless impressed with the landscape of the place; its rolling, steep, abrupt hills and valleys always present some new view to me. The rather quiet day, this, lets me reflect on how all of this terrain got formed. While I know the majority of what I am seeing was nothing more than a thin layer of crushed rock and such deposited under a glacier, I recently started wondering how much of the specific surface that I see is due to just that versus how much has it been altered by the erosion of rain, wind, and snow runoff.

dsc_0051There’s such a great contrast between the flatness of the marsh and the steepness of the hill that rings it. The marsh has filled in over time, first with silt transported by moving water, then by the decaying remains of cattails and such over time.

dsc_0061Coming across the tracks of a Wild Turkey, to me it’s such a good example of how we are able to interpret what has happened previously in a place based on what we presently observe and from those observations, draw some valid conclusions. That it, while I don’t actually see any turkeys ambling by, my past experience with them lets me understand that they did pass by here, and recently, as the tracks are not too obscured.

dsc_0066In the same way, while no living person saw the Ice Ages come and go, the evidence points so strongly to the conclusion that they were here. It’s a model, a picture of what happened that so well encompasses the landscape features, right around here, even, that we can “know” that it did. Once we have that picture in our minds, we can “see” the remnants of the ice that was here. Continued study and modeling further refines that interpretation. This rock here is dark mafic basalt. Our bedrock around here is light colored limestone. So. How did this big rock get here? Comparing its characteristics to rocks in other locations, it’s clear that it came from way up north. If we figure that a lone rock wasn’t just dumped here by a bunch of rowdy Canadians, then it’s pretty reasonable to conclude it was transported by ice, as a rock this size couldn’t be transported any other way. Evidence. Interpretations. Conclusions. Science. Yeah!

dsc_0064Boak Wiesner is a Minnesota Naturalist Volunteer.

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