The St. Croix River has cut a stone window into the ancient past:
“Downstream beyond the dalles, the rocky bluffs that flank the valley are composed of marine sedimentary rocks of Cambrian and Ordovician age. At Taylors Falls… rounded boulders of lava rock of the old shore line of the Cambrian sea are cemented into a massive conglomerate by means of sands and silts that were deposited in the spaces between the boulders. Fossils of Cambrian marine creatures occur in the sandy matrix of the conglomerate. (…) At the village of Marine the river is flowing in the Franconia formation, and more than 100 feet of its sandstone and siltstone beds are exposed above river level. The rocks exposed at the ferry and in a small quarry near the ferry are gray to red-brown, thin-bedded, with gray shale partings. Fossils of trilobites are quite numerous in these rocks.”
-Minnesota’s Rocks and Waters: A geological story (University of Minnesota Geological Survey, U of MN Press, Minneapolis, 1954).
My guess is that these squiggles were once marine worms.