Rock Stories

Reader Contribution by Steve Daut
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I’m a geologist by training. I spent some time in the oil industry until the last bust, then worked in environmental, but I never liked environmental geology all that much. My whole reason for going into geology in the first place was to give me a good excuse to be outside and because I liked rocks. The environmental work I was doing kept me in the office more than the field, and there was nary a decent rock in sight. So I cut loose and got into the nonprofit arena.

But I still like rocks, and our new house has a bunch of really cool big ones. I’m not saying that’s why I wanted to buy the place, although that’s one of the things Sue likes to tell our friends. Well, it’s possible they might have had some influence on me.

I’m not sure if the boulder-sized rocks actually came from the property or if someone brought them in, but a lot of them are boulder-sized (for example, Rock Number 1, below), so it would have taken a pretty hefty piece of equipment to move them.

They actually could have come from the property, because we back up to a hill that was probably a medial moraine when this area was glaciated. Glaciers act like big bulldozers, and a medial moraine is basically the pile of rubble that gets left between two of them bulldozing their way along. Because of this dozing action, the rubble can range in size from sand grains to the largest boulders.

What I like about rocks is that they tell stories, revealing something about their history by what you can see. For instance, Rock Number 2 is basalt (the darker part in most of the rock), which was intruded by granite (the light part on the right side).

Both of these were molten rock that solidified deep within the earth. But how do I know that the basalt came before the granite? Along the edge of the granite where it touches the basalt, the grains are much finer than the rest of it. That indicates that it cooled quicker, giving the crystalline grains less time to grow because they were being cooled by the already solidified basalt. And because of the materials, it’s probable that it was transported down here form Canada. Observing a series of relationships like these allows geologists to slowly sort out the history of an area.

Rock Number 3 is metamorphic, which mean it was transformed through heat and pressure.

The fractures and melting that it shows indicate a violent history, and it contains a type of garnet that is only found in relatively high-pressure metamorphic environments. Most likely, the rock was transported here by the glaciers from the Upper Peninsula of Michigan, where it could have been formed by the same forces that created the rich iron and copper deposits that made mining an economic staple for many years.

Rock Number 4 is a marble.

Unlike the others, that are relatively round, this rock is angular and fractured. That suggests it wasn’t transported very far, because transportation over long distance tends to round out the rough edges, in the same way that a tumbler rounds out a gemstone. But there really isn’t a place locally where such a rock would be formed so this one was probably picked up somewhere else and added to the “collection.”

OK, OK, so I’m a rock geek. And there ARE really a lot of cool rocks on this property. But that didn’t influence me when it came to buying the place. Really. No, I mean really.

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