A very thin extensive tongue of the middle Bangor Limestone in the Black Warrior basin is informally called the Millerella limestone. In MS, the Millerella tongue separates the Parkwood Formation into upper and lower units. The wide extent of the unit indicates rapid transgression over the deltaic sandstones of the lower Parkwood. The lower tongue of the Bangor is overlapped by a northeastward-prograding succession of deltaic sandstones and mudstones of the lower part of the Parkwood. The upper mudstones and sandstones of the Parkwood, which contain several thin tongues of limestone, grade northeastward into the upper part of the Bangor Limestone. The Mississippian-Pennsylvanian boundary is within the Parkwood above the Millerella.

Source: GNU records (USGS DDS-6; Reston GNULEX).