World of Stone II: The Hinckley Sandstone

While on a brief trip up north recently (Moose Lake Agate Days, naturally enough!), I spent an afternoon around Sandstone and Banning State Park. The name of the town is a good clue as to what I saw there.

Why, yes, sandstone!

Sandstone and the long-abandoned town of Banning were big in the sandstone trade. Unlike our old friend the St. Peter Sandstone, the sandstone here has some backbone, enough to make it a sturdy building stone. Quarrying flourished here during the late 19th and early 20th century, then dropped off as tastes changed and the good stuff became more expensive to get at. Banning's quarry operation, which can be visited on an interpretive trail at the park, ended around 1905, though the town puttered along for a few more years. Sandstone's quarrying, now preserved in Robinson Park, ended in 1942.

An old Banning quarry face; note the vertical lines, which are marks made by drills preparing the rock wall for blasting.

Slabs abandoned at Banning; the St. Peter doesn't make slabs like that! (A lot of cool air was sheltered in the various gaps and spaces, making it quite pleasant to walk past on a hot humid day.)

One of the quarry faces at Robinson Park, showing the drill scars.

Durable, but still forgiving enough to be readily carved, as illustrated by Hanley Olson almost 97 years to the day before this photo.

What sandstone is this? It's not something you will see at outcrops in the Twin Cities, although you may find it in buildings. (Pillsbury Hall on the University of Minnesota Minneapolis campus is made of this material.) This is the Hinckley Sandstone, named after nearby Hinckley. Although very well exposed at Sandstone, obviously it wouldn't do to call it the Sandstone Sandstone. Because of its rugged personality, it holds up rapids and waterfalls on its own, making for picturesque settings.

At Big Spring Falls.

Wolf Creek Falls.

Although the soil tends to be a bit thin on top.

So we've had a bunch of pretty pictures. What about the geology?

Okay, one more! Once in a while I nail the composition. Helps to have a good subject, though!

The Hinckley Sandstone outcrops at Banning State Park are great for rambling around, like the basalt outcrops at Interstate State Park. You'll be able to observe a variety of features, some that date from the deposition of the strata, and others that are the result of much later processes. Cross-bedding is frequently visible, as both planar and trough cross-bedding. The trough cross-bedding is interpreted as braided stream deposition, and the low-angle bedding as eolian sand dunes (Boerboom et al. 2002). You may also see beds without bedding, thinly bedded intervals, and rippled surfaces. The composition is almost entirely quartz.

Both kinds of cross-bedding, country *and* western!

There is also a well-preserved fault surface that has left a horst standing (the absence of drill scars is a tell-tale sign that it is natural rather than part of the quarry).

The surface of the horst.

Why is there a horst? It goes back to the Midcontinent Rift. The Hinckley Sandstone is the last thing deposited in the rift before the classic Paleozoic sequence, or at least the last thing that has left behind any rocks (but see below...). As the rift settled and the basalts cooled and contracted, faults formed to release the tension.

Much, much later (much much much later!), following the Paleozoic, Mesozoic, and almost all of the Cenozoic to just past the retreat of the most recent glaciers, the Kettle River scoured potholes into surfaces of the sandstone, and presumably is still doing so today. You can see them in various sizes, including some that are now open all the way to the river.

Looking up into a pothole.

Cross-bedding exposed on the inner surface of a pothole.

Okay, so what about that "but see below..."? The Hinckley Sandstone is a bit enigmatic. Like many sandstones, it is difficult to date from anything intrinsic. You see any fossils or ash beds in those photos? Me neither. All we have to go on is what's above and what's below. What's below is the Fond du Lac Formation, which doesn't really help, and then the Solor Church Formation (again, not helping), and then the basalts from about 1.1 billion years ago. What we find above are either glacial deposits or buried by glacial deposits. The Hinckley Sandstone is traditionally assigned to the Precambrian, but in some ways it resembles Cambrian sandstones found in Wisconsin, such as the Mount Simon Sandstone (Boerboom et al. 2002; Morey 1977; Mossler 2008). It would not be a massive surprise if it turned out to be Cambrian. Interestingly, it was originally described as Cambrian (Winchell 1886), but we shouldn't read too much into that because it was 1886.

References

Boerboom, T. J., A. C. Runkel, and V. W. Chandler. 2002. Bedrock geology of Pine County, Minnesota. Pages 1–20 in T. J. Boerboom, project manager. Contributions to the geology of Pine County, Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 60.

Morey, G. B. 1977. Revised Keweenawan subsurface stratigraphy, southeastern Minnesota. Minnesota Survey, St. Paul, Minnesota. Report of Investigations 16.

Mossler, J. H. 2008. Paleozoic stratigraphic nomenclature for Minnesota. Minnesota Geological Survey, St. Paul, Minnesota. Report of Investigations 65.

Winchell, N. H. 1886. Revision of the stratigraphy of the Cambrian in Minnesota. Minnesota Geological and Natural History Survey, St. Paul, Minnesota. Annual Report 14: 336–337.