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.
University of Minnesota: River Bluffs homepage Archived 2008-09-28 at the Wayback Machine
This article about a specific stratigraphic formation in the United States is a stub.