Rochester, Minnesota — On its last day in the field, Team Minnesota had a geological trip to the Dalles of St. Croix in Interstate State Park, Wisconsin and Minnesota. It was beautiful, and we practically had the place to ourselves, not counting several million mosquitoes.
The Dalles are where the St. Croix River cuts through a series of Middle Proterozoic (1.1 billion year old) basalts. (Dalles is a French word for a narrow river gorge with rocky sides.) The basalts are of the Chengwatana Volcanic Group and represent ten thick flows of lava. They are much more resistant than the surrounding sedimentary rocks, so the river was forced into a narrow, deep and fast channel. In the top image Rachel Wetzel is looking across the river to the Minnesota side.
The basalts have many vesicles (gas bubble holes) that later filled with minerals, producing a structure called an amygdale. (Thanks, Dr. Pollock!) The lava flows vary in the number and size of their vesicles, and the mineralogy of the amygdales. The round white features in this basalt are quartz amygdales. There are also some brownish feldspar phenocrysts (large crystals in the basalt matrix).
This is what a weathered surface of the amygdaloidal basalt looks like. I was at first fooled into thinking it was a sandstone with quartz pebbles! (Such a unit exists unconformably above the basalts.)
There are enormous glacial potholes excavated into the basalt at the Dalles of St. Croix, some as high as 30 meters above the present river. They were formed about 10,000 years ago as glacial meltwater poured across this basalt in volumes many times higher than the river today. Stones would become trapped in eddies and whirlpools, spinning around and grinding their way into the basalt below them. These may be the largest glacial potholes in the world. Etienne Fang shows the size of one. She is sitting on debris, so the holes goes considerably deeper.
Thus Team Minnesota 2016 completed its expedition! Tomorrow the students fly out of the Minneapolis-St. Paul Airport, and Nick and I drive 12 hours or so back to Wooster with our samples and equipment. Our next posts will be about our observations and ideas from labwork back in Wooster.