During early Pennsylvanian time the ancient Apishapa Sierra Grande, a linear mildly positive element trending 45° W. of the present strike of the Sierra Grande uplift, extended across the Raton basin and connected with the strongly positive Front Range element. Seas surrounded the ancient Apishapa Sierra Grande except on the extreme northwest where it connected with the old Front

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Range. Marshall Kay's zeugogeosyncline of Colorado and northern New Mexico bounded the west side of the mildly positive ancient Apishapa Sierra Grande and the east side of the mildly positive Uncompahgre. In early Pennsylvanian time relatively small quantities of arkosic clastics were carried into adjacent seas from the ancient Apishapa Sierra Grande and the Uncompahgre. The Front Range, however, yielded vast quantities of arkosic debris toward seas on the east during Atokan and Desmoinesian time. Orogenic activity along the Front Range is recorded in the coarse clastic Fountain formation which grades laterally into early Pennsylvanian marine sediments as well as late Pennsylvanian marine sediments.

Shortly after Cherokee time a major alteration occurred in the tectonic framework of south-central and southeastern Colorado. The Uncompahgre was strongly uplifted along its east side and the ancient Apishapa Sierra Grande subsided to receive coarse arkosic clastics during late Pennsylvanian time. This writer believes the Sangre de Cristo formation in the Raton basin area is Missouri and Virgil in age, representing a near-source clastic deposit with few interfingering marine limestones. Subsidence of the Raton basin area during Missouri and Virgil time was not uniform. A subsiding east-west trough developed along the New Mexico-Colorado border in the Raton basin, connecting with the north-south subsiding Colorado and northern New Mexico zeugogeosyncline. The present configuration of t e Apishapa and Sierra Grande uplifts developed during Missouri and Virgil time as linear features bounding the more rapid subsiding east-west trough along the Colorado-New Mexico border.

Uplift on the west side of the Uncompahgre near the close of the Pennsylvanian period caused regional tilting toward the east, producing an unconformity at the top of the Sangre de Cristo formation. Rocks of known Permian Leonard age do not interfinger with arkoses of the Sangre de Cristo formation but rest unconformably on Sangre de Cristo rocks and pinch out by onlap along the west side of the Raton basin.

Except for minor erosion during early Triassic time, the Raton basin area received sediments during the Mesozoic era until the late Cretaceous Laramide orogeny. The Colorado and northern New Mexico zeugogeosyncline was the center of strong Laramide uplift, creating the Sangre de Cristo Mountains, and minor Laramide uplift gave rise to the present Apishapa and Sierra Grande topographic features.

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