Glenwood Shale

The Glenwood Shale is a thin Ordivician shale formation in the sedimentary sequence characteristic of the upper Midwestern United States.

Glenwood Shale
Stratigraphic range: Ordovician
Type	Formation
Underlies	Platteville Formation
Overlies	St. Peter Sandstone
Location
Country	United States
Extent	Minnesota

The Glenwood shale cropping out in Minneapolis, Minnesota. In this photo, it is the thin, darker layer that lies between the Platteville Limestone (above: the less-eroded, layered unit that constitutes the majority of the photo) and the St. Peter Sandstone (below: a thin, white stripe of in-place rock followed by a slope of eroded St. Peter Sandstone material).

It lies under the Platteville Limestone 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), it is often ignored in the general stratigraphy.

References

University of Minnesota: River Bluffs homepage Archived 2008-09-28 at the Wayback Machine