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The Carletonian

The Carletonian

A Geologic History of the Arb

<ir="ltr" id="docs-internal-guid-a1ac7776-5461-eae2-f23c-c4719194bdaf">Hopefully the runners and walkers of the Arboretum spend some time observing and enjoying the beauty of its flora and fauna, but my guess is probably not so many people are thinking about what has been underneath their feet the whole way. After all, it is just some dirt on top of a bunch of bedrock, but how did that bedrock get there?

To answer that question we first need to know that the layers of rock that we call bedrock which make up the foundation for the Arboretum are all sedimentary rocks. This simply means that they are composed of sediments (think mud, sand, and gravel) which have turned to rock over millions of years due to the pressure of being buried. Knowing this we can figure out a lot about what happened in this area in ancient history. This is because different sediments are created in different places.

Imagine yourself at the beach (which shouldn’t be at all difficult to do with the balmy weather we’ve had recently) walking into the water. You should notice that the sediments underneath your feet start out as sand, but then they turn into mud. If you were to go even further out to sea you might start to see coral reefs and other aquatic life. It turns out that the sediments at each of these environments turn into different sedimentary rocks. Sands create sandstone, mud creates shale, and the shells and coral reefs create limestone.

If we were to drill way deep down in the Arb and found some sandstone on top of a layer of shale which is on top of a layer of limestone, what would we know about the geologic history of the area?

My answer would be that there was a sea that receded as these rocks were deposited. Since the oldest rocks should be on bottom, and in our example they are the ones created in the deeper sea, we can assume that the water got shallower in this time period. How cool!

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