the madison is formally known as the madison group. in montana, where its thickness reaches , the group is subdivided into the mission canyon formation and lodgepole formation. equivalents of the madison are named the pahasapa limestone in the black hills, leadville limestone (colorado), guernsey limestone (wyoming), and redwall limestone in the grand canyon. the upper part of the madison group, the charles formation in the subsurface of north dakota and northern montana, is not strictly an equivalent of the madison limestone as usually defined.