We’re currently on our way to South Dakota to work on a virtual field trip for Brett’s grant. We stopped in Fairborn, Ohio for a couple of days to test some of our methods at Oakes Quarry Park, including shooting our first gigapan (iOS link). and then continued toward South Dakota. Along the way, we stopped at Pikes Peak State Park, near McGregor, Iowa.
Pikes Peak (which is named after the same Zebulon Pike of Colorado fame) was a regular stop for geology field trips when I was an undergraduate student, so I spent a lot of time camping there in my early 20’s. Before yesterday, the last time I had been at the park was on June 12, 1993, when I proposed to Brett at this overlook:
Behind us the the confluence of the Mississippi and Wisconsin Rivers. As can be seen in the image at the top, the Mississippi River valley is miles wide at this point, and deeply incised into the Paleozoic bedrock, with bluffs around 500 feet high. It’s actually even more impressive than that, since the ancient Mississippi channel cuts another 300 feet below the river’s current surface. This wide deep channel was apparently carved during the Pleistocene, when glacial runoff combined with glacial lake floodwaters to turn the Mississippi into a truly gigantic and fast-flowing river.
The bluffs themselves are made of mostly of Ordovician sediments, which are largely overgrown in the park itself. At Bridal Veil Falls a stream flows over the Platteville Limestone, and eroding the softer Glenwood Shale (I think) underneath. It then continues downhill, carving a steep V-shaped valley into the St.Peter Sandstone:
At the base of the cliff is the late Cambrian Jordan Sandstone, which is also exposed in the Baraboo area and is approximately the same age as the Conococheague Formation. This road cut is a couple of miles north of the park:
It includes some impressive crossbedding:
The regional dip through this area is roughly southwest, so as you go south you get into younger sediments. The Jordan Sandstone goes into the subsurface right around Pikes Peak, so these outcrops represent the southernmost exposure of Cambrian sediments along the entire Mississippi valley.
A good overview of the geology at Pikes Peak State Park is available here.