Sunday, October 30, 2022

Strolling on the Magnolia Member by Hidden Falls

If you're looking for something geological to do in the Twin Cities while we're still under our unseasonably warm and dry weather, may I suggest paying a visit to the new park area above Hidden Falls? [Update, 2022/11/01: this park is called Uŋčí Makhá Park.] As part of the conversion of the former Ford Plant environs, part of the area of the creek into Hidden Falls has been daylighted. The landscaping has produced a mini-bedrock gorge that exposes significant vertical and bedding-plane surfaces of the Magnolia Member of the Platteville and the overlying Carimona Member of the Decorah.

There's nothing quite like this kind of exposure in the Twin Cities; we don't have a lot of exposed non-vertical bedrock in the first place, and this particular stratigraphic interval tends to be out of reach. The closest might be the platform below the overlook at Shadow Falls, but that's more limited in extent and has more of a stair-step profile.

Bonus points for spotting the Deicke K-bentonite.

Many of the exposed bedding plane surfaces reveal the shell beds the Magnolia is known for. The fossils are almost entirely brachiopods (with a few snails) and are represented by dolomitized molds and casts, giving them that characteristic sugary appearance.

See the little bumps? Brachs.

Enlarge for a world of brachiopods.

Here's a closer view showing a few nice examples, representing multiple species.

Also, just for fun, some of the stones used for landscaping are loaded with burrows.

If you stop by, please don't attempt to remove the fossils; it's a park, after all, and the fossils aren't really going to come off in one piece because they're molds and casts. Just enjoy the experience of walking on the seafloor without ever getting wet!

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