PRESQUE ISLE RIVER
The Porcupine Mountains in the northwestern UP have fantastic scenery and interesting geology. The lower stretches of the Presque Isle River have riverbank exposures of upper Mesoproterozoic Oronto Group sedimentary rocks that include the Freda Sandstone and the Nonesuch Shale. The photo below shows some great pothole development in the Nonesuch Shale (~1.07-1.08 billion years) along the Presque Isle River, near the southern shore of Lake Superior.
Nonesuch Shale and huge potholes - lower stretches of the Presque Isle River, Porcupine Mountains, UP of Michigan, USA.
The Nonesuch Shale is famous for being a source of rare Precambrian oil. It also hosts economic concentrations of disseminated copper sulfides and native copper in nearby areas (see pic below).
Native copper (Cu) sheet that occupied a vein or bedding plane in the Nonesuch Shale at White Pine Mine, east of the Porcupine Mountains, UP of Michigan, USA. Copper mineralization occurred at about 1.05-1.06 billion years. Seaman Mineral Museum public display (Michigan Technological University, Houghton, Michigan, USA).
Pothole in the upper Nonesuch Shale at Manido Falls, a little upstream from the first photo above. Potholes enlarge by abrasive grinding action as debris gets swirled around by rushing river water.
Pothole in the upper Nonesuch Shale at Manido Falls, Presque Isle River, Porcupine Mountains, UP of Michigan, USA.
Manido Falls (Presque Isle River). This waterfalls occurs at the contact between the Nonesuch Shale (below) and the Freda Sandstone (above). The Nonesuch here is mudshale and siltstone, while the Freda here is only slightly coarser-grained. Stratigraphically upward, the Freda is a nice sandstone.