Today, I headed out to Franklin Creek State Park. It’s right near my house and has the unique distinction of having the oldest exposed bedrock in the state of Illinois (if the internet is to be believed anyway). Unfortunately, that older bedrock is the same type of rock as the rest of Illinois, sandstone and dolomite.
Anyway, as a geology student living in a very stable, flat, boring piece of craton, you find your places with rocks and end up spending a lot of time around them. This location is one of the places that made me realize my interest in rocks as I decided that geology would be my education path. It’s been fun to return to the same places time and time again each time armed with new knowledge and applying it to what I see around me.
The first time I went there after I knew SOMETHING about geology (but not a lot), I went there thinking that I might find some fossils in limestone. I knew that the area used to be covered in oceans and assumed that there would be a lot of limestone. I would soon discover there is little obvious fossil content, and that what I thought would be limestone is actually dolomite (another, stronger, carbonate rock) and that a large amount of the rock in the area is sandstone.
Today, I know that there are 3 exposed formations of rock representing a wide layer of time exposed at the park– in descending order the St. Peters Sandstone, the Shakopee dolomite and the New Richmond Sandstone. These layers were deposited in the Ordovician (~480 million years ago) and are the types of formations that are key to our understanding that Illinois was once a warm, shallow, tropical sea.
The place in this picture is one of my favorite places to hang around at the park because here, you can very clearly see the disconformity that marks the boundary between the New Richmond and Shakopee formations. It’s hard to see and I’ve never been on top of that ridge, but I think the St. Peter Sandstone is accessible from the top of the rock face too. The stratigraphic column I used in the picture is from a paper about SE Minnesota geology, so it’s not representative of the exact geology of Northern Illinois, but a lot of the same formations make their presence known.
I spent the rest of my time taking cool pictures of the ice.
I still have no real idea of what I want this blog to be about. It would be much longer and take me much longer to write if I were to go as in-depth as I want to, but I think that I want this to be more casual. It will make returning to it day after day a lot easier anyway. If you have any geology questions, fire away and I’ll answer them to the best of my ability.