The aeromagnetic map of the Fossil Springs Roadless Area was compiled from data collected in 1980 by Airmag Surveys, Inc., for the U.S. Geological Survey. Total magnetic intensity measurements were made along east-west flight lines about 0.5 mi apart and flown at an average altitude of 1,000 ft above the ground surface. A regional magnetic field (the International Geomagnetic Reference Field, 1975, updated to months flown) was removed from the data and a constant of 50,800 gammas was added to the adjusted total field intensity values.

The roadless area includes the upper part of Fossil Creek and its northeastern tributaries in Sandrock Canyon and Calf Pen Canyon. The extremely rugged canyon of Fossil Creek is in several places more than 1,500 ft deep. Elevation of the canyon floor ranges from 4,600 ft near the mouth of Calf Pen Canyon to 3,500 ft at the southwest end of the area. Peaks and ridges on the uplands bordering the canyons range in elevation from 5,300 ft west of Fossil Springs to 6,900 ft in the northeastern part of the area. Maximum topographic relief along these uplands is less than 200 ft.

The Fossil Springs Roadless Area is underlain by more than 3,000 ft of Paleozoic rocks consisting mostly of sandstone, shale, dolomite, and limestone. West of Fossil Springs and along most of the border of the area, these rocks are generally overlain by late Tertiary volcanic rocks, mainly basaltic lavas and pyroclastic deposits. Quaternary alluvial, colluvial, landslide, and travertine deposits overlie large parts of the country rock in the canyons. In the southwestern part of the area, the volcanic rocks are more than 2,000 ft thick. Along the northwest and northeast margins, these rocks are 300-400 ft thick.

The prevailing dip of the Paleozoic strata is low to the north or northeast. The dominant homoclinal structure is interrupted by several fault blocks, in which the strata dip westward or southward. All the faults are high-angle normal faults and have displacements commonly ranging from about 50 to 400 ft. Although fault relations are obscure within the volcanic sequence, many faults seem to displace the volcanic rocks less than the underlying Paleozoic rocks. The volcanic rocks, which rest unconformably on the Paleozoic strata, are flat lying in the northern part of the map area, but west of Fossil Springs they dip gently southwestward. The ancestral Mogollon Rim, a north-northwest-trending escarpment of Paleozoic rocks now concealed by Tertiary volcanic rocks lies near Fossil Springs (Twenter, 1962).

The magnetic anomalies and patterns on the aeromagnetic map reflect variations of magnetization in the underlying rocks. Basaltic rocks contain moderate amounts of magnetic minerals, mainly magnetite, and possess strong intensities of magnetization. The more silicic volcanic rocks have much lower magnetization intensities. Sedimentary rocks contain little or no magnetite and are virtually nonmagnetic.