The Permo-Pennsylvanian zeugogeosyncline of Colorado and northern New Mexico, unique among the troughs of the western United States, was a rapidly subsiding basin with highlands on both sides.

Its development was progressive. The Morrow strata were deposited in the northern half of the trough. Following the retreat of the Morrow sea, the Atoka sea entered the southern part of the trough. This sea remained until Desmoinesian time, and the Atoka and Desmoines faunas are mixed. Marine sedimentation reached a climax in Cherokeean time when a seaway traversed the length of the trough. In later Permo-Pennsylvanian time, the sea was confined to the ends of the trough, but thick deposits of nonmarine sediment accumulated in the center of the trough. Upper Pennsylvanian or Wolfcamp strata are several thousand feet thick.

Four principal lithologic units can be recognized: (1) the Belden and Kerber formations of Morrowan, Atokan, and possibly Desmoinesian age; (2) the Clastic member of the Sandia formation of Atokan age; (3) the Minturn and Madera formations, mainly of Cherokeean age; and (4) the Maroon and Sangre de Cristo formations, late Pennsylvanian or Wolfcampian in age. Four minor lithologic units occur at the ends of the trough: (1) the Yeso formation of Leonardian age; (2) the San Andres formation of Permian age; (3) the Weber sandstone of Pennsylvanian and Permian (?) age; and (4) the State Bridge formation of Guadalupian age.

The Jacque Mountain limestone of the Gore area may correlate with a bed which the writer has named the Whiskey Creek Pass limestone member of the Madera formation. If this correlation is correct, this layer can be used as a datum plane over much of the trough.

Cotylosaur and pelycosaur bones occur in the Sangre de Cristo formation in Fremont County, Colorado. These reptiles have not been found previously in this region.

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