The National Park Service has a Passport Program. In any park service gift shop you can buy a passport book, and at the visitor centers there are cancellation stations where you can stamp your book for each park visited. We’ve been collecting NPS passport stamps for several years now, and this has led us to go to some places that we wouldn’t otherwise bother with visiting. This almost always works out to our advantage, as the stops always seem to be more interesting than we anticipated.
After leaving Decorah we drove into southern Minnesota, headed for South Dakota. We noticed that there was a passport site in western Minnesota we had never visited, Pipestone National Monument, which was only 25 miles from the highway. Pipestone is largely a cultural park, emphasizing the pipes made by Native Americans out of locally quarried stone (a flooded quarry is shown above); to a lesser extent, they talk about the efforts at prairie restoration taking place there. We didn’t think our stop would be a long one, but we hadn’t anticipated how much cool geology we would see!
Right at the park entrance was the first site. A series of huge boulders, locally called the Three Maidens (although there are more than three), sitting in a field. How big?
That’s Brett and Tim in the center of the image for scale. Not only are they big, they’re also unusual in composition for western Minnesota, in that they’re all made of granite. The Three Maidens are glacial erratics, Archean granites from Canada carried south by a Pleistocene ice sheet. In fact, the Maidens were all originally a single glacial erratic, that has since broken apart.
The park trail meanders past a surprisingly fast-flowing stream that tumbles over pinkish rocks:
Eventually we arrived at an impressive (for Minnesota!) waterfall, perhaps 4-5 meters high:
The rocks are called the Sioux Quartzite, and it’s pretty interesting stuff. Many of the bedding surfaces are ripples, and cross bedding is visible in vertical sections through the rock:
Did I mention cross bedding?
The Sioux Quartzite is thought to correlate with the similar-looking Baraboo Quartzite in Wisconsin, although the depositional setting may be somewhat different. That makes the Sioux Proterozoic in age, somewhere around 1.5 billion years. Like the Baraboo, it’s a very hard unit and is resistant to erosion, which is how it manages to form these low ridges across the plains (remember, this area has been glaciated). (Incidentally, the layers mined for pipestone are softer metamorphosed clay beds within the quartzite). Most of the erosion occurs as the quartzite spalls off along a mostly north-south oriented joint system, that forms the escarpment for the waterfall:
The escarpment continues for some distance both to the north and the south of the waterfall (this is south of the falls, looking north):
Almost everywhere along this wall there are indications of water flow, including scour holes at its base:
There are also dry, rock-filled channels heading off to the west:
It’s pretty clear that the current waterfall is a tiny remnant of a much larger system. During the late Pleistocene, as the Laurentide Ice Sheet was melting, water was pouring through this area, pouring over the Sioux Quartzite in a mini-Niagara Falls that were at least a mile wide, if only four meters tall!
We ended up spending more than four hours at Pipestone, which completely messed up our schedule for the day. But it was well worth it to unexpectedly find ourselves standing at the base of this dry waterfall, imagining the Pleistocene vista.