Layers of the Ledges
March 12, 2014
The Earth Science class for educators at Michigan Tech has an online textbook on Michigan Geography & Geology that’s pretty cool. The chapter on the Ledges at Grand Ledge includes At the Edge of an Ancient Ocean that talks about the rocks that make up The Ledges and begins:
The rocks at Grand Ledge are significant for several reasons. Grand Ledge is an “oasis” of bedrock in an “ocean” of glacial drift that blankets the Lower Peninsula, providing geologists a window into the distant past. The diverse set of sedimentary rocks contains a wealth of information on the plants and animals that dominated the Pennsylvanian Period, about 320 to 290 million years ago. The characteristics of the rocks allowed geologists to reconstruct the changing environment that marked the demise of a great inland ocean. The rocks have been quarried and hold economic value. Lastly, Grand Ledge is scenic and enjoyed by hikers, paddlers, and climbers.
Nearly all students of Michigan geology make a pilgrimage to Grand Ledge at some point in their careers. Good exposures of sedimentary rocks are rare in the Lower Peninsula. Not only are the rocks well exposed but they offer an opportunity to test your skills in identifying a variety of sandstones, some shale and limestone, and even 2 coal. The rocks are exposed in a few abandon quarries and in exposures along the Grand River. To get a good look at the rocks you will need drive between exposure north and south of the river. But don’t be discouraged; the distances are short.
As always in geology, the best place to start is at the base of the stratigraphic section, the oldest rocks. The lower part of the section contains shale, siltstone, and type of sandstone called greywacke. The shale is gray and so fine-grained that you cannot see the mud-sized particles that compose it. If you are brave, you might put a tiny piece in your mouth and push it around a bit. Shale feels smooth, almost creamy, a result of the mud. The shale is also soft and erodes to relatively gentle slopes. Shale is exposed at the base of the layers at the Face Brick Quarry. Think of the light-colored siltstone as a silty shale. You might rub the rock against your thumb and see if any small, visible grains come loose. Again, a taste test might be in order. Siltstone will leave a 3 gritty feel in your mouth. Siltstone is exposed at the base of the rock layers at the American Vitrified Quarry. The greywacke is a greenish-gray colored sandstone and the sand grains are visible to your unaided eye, no tasting required. With a hand lens you can see the rock is made of a mixture of sand sizes, what geologists call poor sorting, and a variety of sand compositions, including quartz, feldspar, mica, and fragments of pre-existing rocks. Greywacke is exposed just above the beach at the Face Brick Quarry.
More winter wallpaper on Michigan in Pictures.