Late Tertiary basaltic dikes intrude ancient Proterozoic gneissic rock in the Gila Mtns. along US I-8, 15 miles east of Yuma. The bedrock (gneiss) here is 1.6 to 1.8 billion years old, but the mountain range formed just in the past 30 million years as part of crustal extension in the Basin & Range Province.
Online resources:
Richard, S. M., Ferguson, A.C., and Skotnicki, J. S., 1997, Geology of the Fortuna Mine, Yuma County, Arizona. Arizona Geological Survey Open File Report, OFR-97-16, 13 p. http://repository.azgs.az.gov/uri_gin/azgs/dlio/966
Interactive Geologic Map of Arizona (1:1,000,000 scale) http://data.azgs.az.gov/
