Baraboo geology, Part 2

Part 1 is available here.

On the second day of our Baraboo geologic excursion, Tim and I left the hotel early and headed to Stop 7, a famous locality called Van Hise Rock. Another car was already in the parking area next to the rock, and we introduced ourselves to Dr. Mengist Teklay Berhe, a geologist at the University of Wisconsin-Baraboo/Sauk County who was scouting field trip stops for his geology class. Van Hise Rock was a natural stop for him, because how many rocks get to be named National Historic Sites?:

Here’s the rock itself:

There’s our pink quartzite again, showing up on the right side of the picture. It’s also visible on the extreme left, looking a little less pink because it’s in the shade. There’s also an intervening layer of phyllite, a metamorphosed shale. What remarkable is that, while the quartzite we saw at Stops 1 and 6 was dipping to the northwest at about 20 degrees, these rocks are standing almost vertically. There are also interesting cleavage fractures visible in both types of rock, angling down about 45 degrees in the phyllite, and close to horizontal in the quartzite. These develop in response to the stress applied to the rocks during the folding event that caused them to stand on end (I’ll talk about this more in Part 3). There are also other indications of the stress these rocks endured, such as these en echelon tension gashes in the quartzite:

Accompanied by Dr. Teklay Berhe, we left Van Hise Rock and walked along a trail through Ableman’s Gorge to Stop 8, an abandoned quarry that provided us another spectacular site:

A near-vertical bedding surface in the quartzite, covered with ripples! The exposures in the quarry also gave us an opportunity to take some additional orientation measurements. Along the trail, looking through the trees we noticed another rock at the top of the cliff above the quartzite, too high for us to easily reach (indicated by the arrow):

Taking advantage of my camera’s zoom lens, we tried to get a closer look:

It’s hard to be certain from a distance, but these look a lot like the sandstones and conglomerates we saw in Parfrey’s Glen at Stops 2 and 3. You can even see what may be quartzite clasts (near the out of focus yellow leaves in the foreground).

Parting ways with Dr. Teklay Berhe, Tim and I headed to Stop 9, a road cut on the side of (very busy) Highway 12:

More quartzite, and once again with a shallower dip more comparable to Parfrey’s Glen and Devil’s Lake than to Van Hise Rock. Up close, extensive cross beds are visible:

The ripples can also be seen on the bedding surfaces:

We only had a few minutes at Highway 12, because we needed to get to Stop 10, the Kraemer Co. La Rue Quarry (I contacted Kraemer in advance to arrange access to the quarry, as it is currently active).

La Rue is a quartzite quarry, and the same cross-bedded quartzites seen at our other stops are visible here:

At the top of the quarry walls, the dipping quartzite beds are overlain by a flat-lying sandstone, with large clasts in the sandstone just above the contact:

The contact is even more dramatic in a different part of the quarry:

Here’s the annotated version (although the placement of some of the lines is approximate, since we couldn’t get too close to the wall):

The flat lying sandstones are lapping onto the heavily eroded top of the older quartzite, which is dipping to the northwest (left in this picture). Much of the contact between them is a breccia, a sedimentary rock made of of large, angular clasts. In this case, the breccia contains large chunks of quartzite, some approaching the size of a car.

The La Rue Quarry was the last stop on our geology excursion, and after lunch we continued on to Minneapolis for the GSA meeting. In Part 3, I’ll try to put all of this information together to build up a geologic story of the Baraboo region.

I’d like to thank Mengist Teklay Berhe for his pleasant company during our trip, and Mark Sander of Kraemer Co. for providing access to the La Rue Quarry.

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