A synclinal area between the Medicine Bow and Park ranges demonstrates several principles in the tectonic history of the southern Rocky Mountains. The area is of interest because: (1) it illustrates the episodic nature of tectonic events in this region; (2) the succession of fossiliferous Tertiary sediments are unusually complete and provide excellent clues to the timing and nature of these events; and (3) several structural elements in this area have trends independent of the regional structural patterns. Early Tertiary phases of tectonic activity were compressional in origin, but late Tertiary phases were both compressional and tensional.

An early Tertiary compressional phase produced the bold outlines of the present ranges and basins and induced erosion of all sediments from the structurally elevated areas. Debris which formed the Paleocene Coalmont and equivalent formations filled the basins and overlapped the truncated edges of older formations. North Park basin continued to sink following this deposition.

A second compressional episode involved Paleocene sediments in the movements, and culminated with extensive faulting during Eocene time. Although most of these thrusts parallel range flanks, the Independence fault which forms the north boundary of North Park basin displays evidence for movement of over 4 miles in a direction transverse to previous structural trends. The position of this fault within a complex zone between two regions possessing opposite structural asymmetry may explain, in part, its diverse trend.

Normal faults, warps, and folds affecting late Miocene rocks, and probably concurrent with regional uplift, distinctly modified the geologic regime during a third tectonic episode. Horsts rose 800 feet along range flanks as adjacent wedges of late Miocene rocks were downfaulted and preserved in linear troughs. The North Park syncline formed athwart the axis of North Park basin and transected some pre-existent north-trending range flank structures. These late Tertiary movements should be stressed because they emphasize the probability that similar movements modified adjacent areas where late Tertiary sedimentary sequences are either less complete or absent.

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