The Glenwood Shale is a thin Ordivician shale

Shale

Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...

 formation in the sedimentary sequence characteristic of the upper Midwestern United States. It lies under the Platteville Limestone

Platteville Limestone

The Platteville Limestone is the Ordivician limestone formation in the sedimentary sequence characteristic of the upper Midwestern United States. It overlies the thin Glenwood Shale, which overlies the thick Saint Peter Sandstone...

 and above the Saint Peter Sandstone. Together, these three units represent a sequence of sea-level rise during Ordovician time. Because it is often very thin (~10 cm or less in the Twin Cities

Twin cities

Twin cities are a special case of two cities or urban centres which are founded in close geographic proximity and then grow into each other over time...

), it is often ignored in the general stratigraphy.