The Monument Spring area of the Marathon uplift, was analyzed using a Kelsh plotter and NASA high-altitude color infrared (CIR) imagery.

Structurally, the Marathon uplift is a broad dome from which the Cretaceous cover has been eroded, exposing the Paleozoic structures. These Paleozoic rocks, ranging in age from Late Cambrian to Pennsylvanian, are complexly deformed and exhibit a variety of structural attitudes.

The Marathon region was originally mapped extensively by P. B. King in 1937. The Monument Spring area contains two of the most prominent structural features described by King: the Marathon anticlinorium and the Pena Colorada synclinorium. These features are characterized by tight folds and thrust faults striking in a northeasterly direction. North-trending shears are also found within the area.

Although there is a general agreement between present maps and King's original interpretation, the use of high-altitude specialized photography and quantitative Kelsh data provides additional information on the structural complexities of the area. This information, derived from the high-resolution model (obtained with the Kelsh Plotter) is perhaps not readily apparent in the field. The interpretations derived from this information are outlined in the detailed mapping of the Rock House Gap, Sunshine Spring area, and one additional area southeast of Monument Creek.

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The high-resolution, infrared imagery and analytical photo-grammetric data in combination with more traditional geological information has proven to be a useful tool in this complex structural area.

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