The Paradox basin, major area of tectonic subsidence in the Four Corners region, is, with its broad shelves and sediment-supplying highlands, more than 360 miles long in a northwest trend and about 180 miles wide in a northeast direction. Sediments of the Pennsylvanian system comprise the Atokan-early Desmoinesian shelf carbonates and fine clastics of the Molas and Pinkerton Trail formations; the Cherokee penesaline-saline-penesaline megacycle, interbedded with fine- to medium- and coarse-grained clastics from northeasterly sources which make up the Paradox formation; the Missouri to Virgil shelf carbonates grading westward into high-shelf dolomite and fine to medium clastics, and grading northeastward into coarse arkosic clastics to form the Honaker Trail formation; the ico transition facies from marine to non-marine conditions and encompassing beds which range in age from Desmoinesian (Marmaton) near the uplifts to Wolfcampian Hueco in the accessways to the southeast and northwest through which the latest Paleozoic seas withdrew from the region.

Results of this investigation establish the existence of an important datum (the "P" datum) which is the top of the Paradox formation. This datum, over a great part of the region, is a disconformity which marks the end of Paradox penesaline deposition, and is a mappable surface critical in the evaluation of the sedimentational and structural history of the Paradox geosyncline.

As new drilling proceeds in the wildcat areas of the region, the Pennsylvanian system will need to be subdivided into smaller time-stratigraphic units which are correlative with practical lithic units here named, so that they can be used with confidence by geologists working in the Four Corners region.