A complex anticlinal flexure about 70 miles wide extends northeastward from central Bent County into Kit Carson County, Colorado. This fold, called the Las Animas arch, appears on structural maps of post-Permian beds to be the north-plunging nose of the Sierra Grande arch of northeastern New Mexico. But pre-Pennsylvanian structural maps show that the Las Animas and Sierra Grande arches are separated by a trough which was progressively downwarped in Pennsylvanian and Permian time. The Las Animas arch developed its anticlinal structure only because of relatively greater downwarping of regions on its flanks. Sedimentary formations extend across the arch without local facies change and with only gradual, unidirectional changes of thickness.

From pre-Cambrian through Mississippian time shallow-water-type marine beds were intermittently deposited uniformly over the arch area. In late Mississippian time these beds were tilted eastward and slightly beveled. This was followed by downwarping south and west of the arch in the Pennsylvanian and Permian. In Triassic and Jurassic the arch and the region about it had a mildly negative structural tendency and then became part of the Cretaceous geosyncline, receiving sediments similar in thickness and type to those of surrounding regions. When the Denver basin and the Hugoton embayment were formed in late Cretaceous or Tertiary time, or both, the Las Animas arch retained its positive tendency relative to the basins.

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