What was mid Michigan like during the Pennsylvanian period?
We must first go back 500 million years when the sedimentary rocks that now make up the Michigan basin were first deposited. Throughout the Paleozoic Era roughly 14,000 feet of sedimentary rocks accumulated. The youngest of these rocks are exposed to the west of Saginaw Bay while the rocks at the surface become older as you travel outward in every direction. These somewhat circular bands dip on an average of 60 ft/mi toward the center of the basin. These rocks have not been subject to any metamorphism or igneous intrusions. During the Pennsylvanian period (320 to 286 mya) water covered much of Michigan. The sand, silt, and clay deposits that make up “the ledges” were carried in stream channels and deposited along river banks and beaches. Compression and time cemented the sediments into the sandstone that we see today. The tiny quartz crystals that make up the sand are held together by calcite. The calcite is easily dissolved by water so these rocks are subject to a lot of weathering. T
