Earth Geology Rock Types Shale
|This photo is taken along the Macomb access ramp to Route 60, heading east. The top of the white post marks a narrow band of Northview Shale. |
Shale is a sedimentary rock composed of very small to silt-sized particles of clay. Shale results from the deposition and compaction of fine-grained material (mud) on the bottom of a quiet sea floor. Relatively minor amounts of shale occur within most of the carbonate rock formations as thin layers and lenses. However, in the northernmost part of the Bryant watershed is a shale layer that is ten to twenty feet thick or more. This is a layer called the Northview Shale, which is Mississippian in age and occurs between the Compton and Pierson limestones.
|A closer view of the shale shows that it is made up of thin layers. The band is between 6 and 10 inches deep. |
Shale is by nature a relatively soft rock that is easily eroded and seldom seen in a natural outcrop. Northview Shale is exposed in a roadcut along Highway 60 in southwestern Wright County. At this location the shale is a grey to greenish grey crumbly rock that is deeply eroded along the roadside. The shale contains numerous thin veins or fracture fillings of brown iron oxide (limonite). In some localities the shale contains numerous fossils that have been replaced by the limonite.