Tuesday, August 17, 2010


Over the Memorial Day weekend I had a free day while visiting relatives and decided to go fossil collecting near Kremmling, CO. It's a small town in the mountains about 2 hours west of Denver. My Grandfather used to go hunting for Elk up in this area years ago but I wasn't planning on going that high or far into the mountains.

The weather was nice with temperatures in the mid to upper 70's but the higher elevations of the mountains still had snow on them.

The area I was planning to explore is in the Pierre shale which is equivalent to the Mancos shale found farther west in Colorado and Utah. It's Cretaceous in age and represents a shallow sea that once covered the middle part of the North American continent. At Kremmling the layers are mostly flat but then turn up where they intersect the base of the mountains. Below is a scan from the Geologic map of the area.

The pale green colored area is Pierre shale and you can see some marker beds that are called out as well.

I've been up here before but I'd done more research this time and had a better idea of what and where to look for fossils. I stopped in an area that has one of the more productive marker beds and started to climb the hills. The marker beds are often composed of sandstone and thus form the ridges of the hills. You can find fossils weathered out and lying on the slope of the hill as well as blocks of the sandstone that have tumbled down from above. Most of what you find are the shells of the pelecypod Inoceramus. This is a prolific genus within the Pierre shale and corresponding layers around the world like the Gault Clay in London. Some became very large, about the size of dinner plates, but most that I found are smaller.

In some cases the shells formed a near pavement on the bottom of the ocean:

Depending on the layering of the rock, you could see some worm tubes preserved as well:

Also present throughout the layer are Baculite fragments. This was an Ammonite that had a long, linear shell with one end curled into a spiral. the other end was where the animal lived. They looked very similar to the orthoconic cephalopods found in the Paleozoic but are only distantly related. Besides the shape of the animal, the next most obvious detail is the suture pattern of the back chambers.

The real prizes are the Ammonites and I was lucky enough to find two. Below is a small, Scaphites with some shell material preserved.

My best find was this 4" Placenticeras but I had some trouble getting it out of the rock. I'll discuss this specimen more in another post.

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