The Presque Isle River has riverbank exposures of sedimentary rocks that include Copper Harbor Conglomerate, Freda Sandstone and Nonesuch Shale. These sedimentary rocks are approximately 1 billion years old. The Nonesuch Shale at GZ contains layers of gray to black shale, siltstone and sandstone.
These round potholes form where an eddy current swirls pebbles and grains of sand in a circular path. Over time, the swirling action of these materials wears away the softer shale, forming the large, smooth-sided potholes seen here. This is a type of physical weathering known as corrasion.
Source: Michigan DNR
Prior to logging this cache, email the answers to the following questions:
- Estimate the diameter of the largest pothole?
- Can you notice water swirling around in any of the potholes?
- Some of the potholes are missing a side. Does the water still swirl around in these?
- Optional: Post a photo of your visit.
This EarthCache is located in Porcupine Mountains Wilderness State Park. There are quite a few stairs down to GZ, and therefore quite a few to get back to your car. Some of us feel this is a physical challenge that warrants a higher terrain rating. The Presque Isle River has been designated a National Wild and Scenic River.