Sorry folks, but people keep adding these awesome shots of Petit Portal in the Pictured Rocks National Lakeshore. Near the top of my personal Michigan Bucket List is being where this kayaker is.
Here’s a shot from a place on my Michigan kayaking bucket list – Petit Portal (also known as Petit Arch and Arch Rock by some) and other cliff formations of the Pictured Rocks National Lakeshore. The Lakeshore’s Geologic Formations page begins:
The geologic formations of Pictured Rocks National Lakeshore are most spectacularly represented by the 50-200 ft. sandstone cliffs that extend for more than 15 miles along the shoreline. Sea caves, arches, blowholes, turrets, stone spires, and other features have been sculpted from these cliffs over the centuries by unceasing waves and weather.
The name “Pictured Rocks” comes from the streaks of mineral stain that decorate the cliffs. Stunning colors occur when groundwater oozes out of cracks and trickles down the rock face. Iron (red and orange), copper (blue and green), manganese (brown and black), and limonite (white) are among the most common color-producing minerals.
Geologic history recorded in the sedimentary rocks and surficial deposits of Pictured Rocks National Lakeshore is limited to two widely separated intervals of geologic time, the Late Precambrian, Cambrian, and Early Ordovician Periods (500-800 million years before present), and the Late Quaternary Period (two million years before present to the present).
You can read on for more about each geologic era, and I think that that this report by Lakeshore Volunteer Geologist Robert Rose (pdf) has some graphics that really help to understand how the layers fit together.