by Robert Regis
Having grown up in the U.P., I was intrigued by the rocks and minerals and the spectacular rock outcroppings seen in my nearby travels. One such place is Pictured Rocks National Lakeshore. There you see rusty red and orange sandstone rising abruptly from the deep blue waters of Lake Superior, making a dramatic visual contrast. Beautiful indeed, but the young scientist in me asked where did those sandstones come from? And what makes them so red? Why are the cliffs so dramatic?
Turns out the sandstones are quite young, compared to other rocks around the U.P. They are a mere 500 million years old! They were deposited along an ancient shoreline, with streams depositing sand across a gentle, rocky plain. Except at that time, the PRNL was located just south of the equator. That’s right, the equator. There were no palm trees lining the beach, though, for land plants had not yet made their appearance on Earth. It was a barren scene.
The rocks tell a story of change over time, like pages in a book. At the base of the cliffs, right at or below water level, is the Jacobsville Sandstone, named for sandstone quarry owner John Henry Jacobs. There, the rock was quarried for building stone, and may be seen in many buildings around the U.P. (Marquette Courthouse and the Cathedral, etc). Above the Jacobsville Sandstone is the Munising Formation, which forms most of the vertical cliffs. The lower member is the Chapel Rock Sandstone, and above it is the Miners Castle Sandstone. Although similar, the sediments that make up the Chapel Rock Sandstone came from a different source than the Miners Castle, and hence have a slightly different appearance. The structures and minerals in the rocks show the sediments first came from highlands to the south (Chapel Rock) and later from the east (Miners Castle) as seas encroached on the land and became deeper and deeper. The red color comes from hematite, which stains the sediments. Above the sandstone cliff is a bed of dolomitic sandstone, which indicates that the deepening ocean was warm but still shallow. Dolomite forms a resistant layer that is difficult to erode, and is the “caprock” that protects the layers below. It is responsible for the many waterfalls and abrupt topography in PRNL because streams have difficulty eroding through the layer.
Moving west toward Marquette, you can see the Jacobsville Sandstone again at Presque Isle Park. In some places, the sandstone has lost its red hematite coloration by chemical leaching and is now white.
The name Presque Isle means “almost an island.” In fact, not too many years ago (to a geologist) the park was an island. You can observe the old shorelines of glacial Lake Nipissing (pre-Lake Superior) from about 5,000 years ago at the bandshell and the gazebo near the entrance to the park. The bluffs were formed by waves eroding into the island when the lake was about twenty-five feet higher. An underwater ridge of sand developed between the island and the mainland, and when the lake lowered, the ridge became a land connection which geologists call a “tombolo.” The road to the Park is on the tombolo.
Underneath the sandstone is rock that locals refer to as “Black Rocks.” The Black Rocks are a metamorphosed igneous peridotite about 1.7 billion years old! The rock was exposed previously, because the Jacobsville Sandstone rests directly on top of that ancient erosional surface (called a nonconformity). You can see this nonconformity at many places in the Park, but probably best along the west side, south of “Sunset Point.”
Robert Regis has been a geology Professor at Northern Michigan University for over twenty years. His degrees are from NMU, Indiana State University, and Michigan Tech University. He has published and presented numerous articles on the geology of the U.P.
Reprinted with permission from Health & Happiness U.P. Magazine, Summer 2012.