The so-called Trenton and Galena limestones of Wisconsin, Illinois, Iowa, and Minnesota have long been recognized as forming a practically continuous surface area, and they are so represented on geologic maps (figure 1). They are nearly coextensive with the subjacent Saint Peter sandstone. All three of these formations are greatly cut away by erosion on their northward side and they appear to have originally extended much farther than they now do in that direction. The extent of area from which these formations of the Ordovician age have been denuded is perhaps as great as their present surface area. This area was also once doubtless covered, in part at least, by the Maquoketa shales and by Silurian and Devonian formations. On the general southward side of the present surface area of the so-called Trenton and Galena limestones, these formations, with the overlying Maquoketa shales, extend with gentle dip very far, . . .