With the close of the Triassic, we come to the Sundance Formation, which dates from the Jurassic period. Outcrops of the Sundance can be seen throughout Wyoming, including the south side of Muddy Mountain, but the best place to see the Sundance is undoubtedly at the Cottonwood Creek Dinosaur Walk trail near Alcova.
The trail starts on an outcrop of the Jelm, which dates from the Triassic period. By the time you get to the end of the trail, at the top of the hill, you’re in the shadow of an outcrop of the Lakota Conglomerate, from the beginning of the Cretaceous period. With the simple act of hiking up a hill, you’ve traveled through 90 million years of geological history and seen a sea invade Wyoming from the northwest, cover it in fairly deep water, and then retreat again, leaving Wyoming high and dry.
When I first heard of the ‘Sundance Sea,’ I thought that maybe the name described the way that the light must have glinted and sparkled on the wave tops. Alas, no – it’s just named after the formation, which in turn is named after the town of Sundance, which itself is ultimately named after an extremely painful native American ritual involving thirst, exhaustion, and skewers piercing the muscles of the chest.
The really cool thing about the Sundance, at least for a fossil nerd like me, is the fossil fauna. Near the end of the Jurassic almost all of Wyoming, as well as large portions of several neighboring states, were covered by the Sundance Sea, perhaps to a depth of a couple hundred meters. The most common fossils in the Sundance are belemnites. Belemnites are hard bullet-shaped internal shells, or ‘guards,’ from squid-like creatures called Pachyteuthis. Most of the belemnite animal was soft and squishy, like a modern squid, and things like the fins, the arms and the eyeballs would have rotted away and left nothing. But those hard shells were really durable and they survive in unbelievable quantities in the Sundance – there are places where you can find hundreds of the things weathering out of riverbanks and hillsides. Nowadays Wyoming may be where the deer and the antelope play, but 160 million years ago this place was Squid City.
Another abundant fossil critter is the crinoid, an animal that was shaped like a flower. After a crinoid died, its stem broke apart into tiny star-shaped sections which were scattered, buried, and eventually fossilized. There are a couple of species of ant that collect these tiny stars and bring them back to their anthills. I can’t walk by an anthill without stopping for a close look – every now and again I find one that looks like a pile of Lucky Charms! In addition to the stars, the ants will also collect tiny sharks’ teeth and baby belemnites. Scavenging these little fossils from anthills is a whole lot easier than crawling around on my hands and knees trying to find them myself. Every time one of my students starts kicking apart an anthill I tell him ‘Stop! Don’t do that! Those ants work for ME!’
The real prize for a Sundance fossil hunter is an ammonite. In life, these creatures looked like a snail with the head of an octopus, with a beautiful spiral shell. There are formations that are just chock full of ammonites, but the Sundance isn’t one of them. You occasionally find bits and pieces of their shells, but complete specimens are rare. I’ll never forget the first time I found a complete ammonite in the Sundance – it was only a little bit bigger than a silver dollar, but I was still so excited that I broke into a rousing rendition of Queen’s ‘We Are the Champions’ right there on the outcrop.
Join the Tate Geological Museum on December 9 from 10 a.m.-2 p.m. for its annual Holiday Open House. Meet Santasaurus, make an ornament, learn about Wyoming’s fossils, and have some punch and cider!