Note: This is an Earthcache. There is no physical cache to find. Logging this Earthcache requires that you undertake an educational task relating to the specific Earth Science at the site.
Prior to logging this cache, click on Message this owner, or send an email with 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?
- Required to log this cache:Please provide a photo of yourself or a personal item in the picture to prove you visited the site. Upload the photo with your log.
Presque Isle River
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
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.
