World of Stone

This is part of Devil's Parlor in the Minnesota side of Interstate Park, on the St. Croix River. I was at the park on Saturday for a park geology program, and following work I wandered through the Glacial Gardens section. Interstate Park, as its name suggests, has units in two states, in this case Minnesota and Wisconsin. The two parts are state parks, associated with but not part of St. Croix National Scenic Riverway (the Wisconsin side is also the western trailhead for the Ice Age National Scenic Trail, as well as one of the nine parts of the Ice Age National Scientific Reserve). Interstate Park is known for its glacial landforms, imprinted primarily on Precambrian bedrock.

The St. Croix Valley is underlain by a much older feature: the Midcontinent Rift, a 1.1-billion-year-old scar in the central North American continent. This attempted fissure winds between Kansas and Michigan, being most visible in the Upper Midwest. When a rift is forming, great amounts of magma can reach the surface, resulting in thick layers of hard basalt. Interstate Park has large outcrops of rift basalt forming the gorge of the river and the platforms overlooking it.

Looking downstream from Angle Rock.

Above the basalt are some Cambrian rocks, but Ordovician rocks are lacking, and this area has been interpreted as an Ordovician island. The Cambrian rocks along the St. Croix have yielded invertebrate fossils such as brachiopods, trilobites, and snail-like mollusks, and historically speaking these are among the first Cambrian fossils known from North America, having been first described in the early 1850s. The Cambrian rocks (particularly what was once known as the Franconia Formation but is now called the Tunnel City Group) are not exposed in the Glacial Gardens, but are exposed nearby (quite well on the US 8/MN 95 roadcut coming into the park from the south).

The "glacial" part of the story comes in during the Pleistocene. Each glacial advance would bury the preexisting drainage, and after each retreat rivers would reform, usually creating a new course that combined newly eroded cuts and re-occupation of old channels. In the St. Croix Valley, the Cambrian rocks were eroded easily enough, but below that the basalt posed more of a challenge. Between the two parts of Interstate Park, the river encountered an area of jointing (fractures) in the basalt that could be eroded more readily than the surrounding basalt. This jointing zone also happened to be at a right angle to the channel the river had been carving. Thus, Angle Rock (and its history of log jams).

Looking upstream from the angle.

Before the river made its deep gorge, it swept across a wider area at a higher elevation, including the Glacial Gardens. One of the results was the creation of glacial potholes. The river had probably formed a series of rapids, and sometimes rocky debris would get caught in eddies. The rocks would tumble around in a small area, eventually drilling out a hole with enough time. Interstate Park has both the largest concentration of glacial potholes and the deepest known potholes in the world.

Looking up through the Bake Oven.

Into the Bottomless Pit.

Slightly less-bottomless pits. Potholes are all over here; it's the one place in Minnesota where we don't mind them.