Geology of the Eagle Mountains, Hudspeth County, Texas
— James R. Underwood Jr.

Eagle Mountains and the associated highlands, Devil Ridge and Indio Mountains, trending generally northwest to northnorthwest, are surrounded by "bolsons" or intermontane basins partly filled with alluvium and are an extension into southeastern Hudspeth County of the eastern part of a long, narrow, mountainous, structural belt that begins some 240 km southeast near Ojinaga, Chihuahua, Mexico. The summit of Eagle Peak, 27 km southwest of Van Horn, Culberson County, 38 km southeast of Sierra Blanca, Hudspeth County, and 2,285 m above sea level, is the highest point in Hudspeth County. Lowest point in the map area, the southeastern corner along the Rio Grande, is 960 m above sea level; the maximum difference in elevation is about 1,325 m.

In the highlands, well-exposed rocks ranging in age from Precambrian to Recent include about 1,525 m of metamorphosed Precambrian sedimentary rocks, at least 300 m of Permian limestone, and about 2,135 m of marine Cretaceous strata, much of which is covered by an early Tertiary volcanic rock sequence of flow breccia and fine-grained flow and pyroclastic rocks. Small intrusive bodies, mainly sills and dikes, crop out throughout the area; the highest part of the Eagles is a small, roughly semicircular stock. The range is flanked by well-defined alluvial fans and terraces.

The stratigraphy of the Cretaceous rocks was controlled by a fluctuating shoreline during the general advance of the Cretaceous sea north and eastward from the Chihuahua Trough onto the Diablo Platform and the continental margin; structurally, the spatial relationships of the Cretaceous rocks were controlled by their location (1) on the southwest flank of the Van Horn uplift, (2) near the northwest end of the Laramide Chihuahua tectonic belt, and (3) near the eastern margin of the Basin and Range Province.

The area is in the Mexican Highlands section of the Basin and Range Province; the highlands of the Eagle Mountains and vicinity are horsts flanked by intermontane basins or grabens, all of which were created by mid- to late Tertiary block faulting that followed the volcanic activity. This deformation was superimposed on earlier Laramide folds, thrust faults, and strike-slip faults characteristic of the Chihuahua tectonic belt. The thickness of the alluvial fill in the fault-created intermontane basins is not certainly known, but several deep water wells are believed to have bottomed in the alluvium; at the head of Green River, the fill is at least 335 m thick; at Hot Wells, at least 300 m thick; and near Red Light windmills, at least 200 m thick. In the closed basins, the drainage systems are responding to a rising base level, whereas the open drainage systems are operating under a regime in which base level is dropping. Basin integration is advanced.