Overpass Quarry

The upper part of this formation is Decorah Shale. The limestone is in the Platteville Formation, both part of the Ordovician Age. Both of these rock formations have numerous fossils. We came here with the express purpose of collecting fossiliferous limestone. Many of the fossils are small "shell" type but there are also larger fossils, especially the cephalopods that can reach six feet in length. A full cephalopod fossil is very rare.

Notice in the photos the layering effect of the rock. These sedimentary rocks were deposited in layers from the remains of sea life. Notice that the layers vary in thickness. The varying thiskness of the layer depends on the depth of the water at the time of the formation of the layer. Each layer is an indication of a local climate or geologic change. In addition to the thickness change from the water is the rock type change. In shallow water larger particles like sand are deposited and in deep water very small particles of soil are deposited to form shale (that may be metamorphosed into slate). The intermediate depths are where the carbonate rocks are formed. Within the layers of carbonate rock the thickness variations continue to reflect the change in the depth of the water where the thickness of the layer is controlled by the depth of the water where the required sealife can form the rock. The cyclic variation in the thickness of the rock layers as well as the variation of sandstone, limestone and shale are indications of the changing water depth over time.