The Spanish Peaks are geologically distinct from the faulted and uplifted mountains of the Sangre de Cristo range to the west. To the geologist, the Spanish Peaks are prime examples of stocks—large masses of igneous rock layered by sedimentary rock and exposed by erosion.
Stocks
27 million years ago, pressures and stresses built up by continental drift movements caused cracks to form in the area’s sedimentary formations. Molten rock from the mantle beneath the Earth’s crust began to surge upward into the lower areas of these cracks about 25 million years ago. When the magma cooled and hardened beneath the Earth’s surface, it formed large horizontal batholiths of granite called stocks. These are what form the Spanish Peaks.
Several miles of sedimentary rock covered the Spanish Peaks’ stocks. Over the last 25 million years, uplifts and folds raised the surface of the land while elements eroded away the softer, overlying sedimentary rock and exposed the underlying hard, igneous stocks of the Spanish Peaks.
Some of the area’s stocks include Mt. Mestas, Silver Mountain, Sheep and Little Sheep Mountains, and Iron Mountain.
Dikes
Among the most unusual features of the Spanish Peaks are the great dikes radiating out from the mountains like spokes of a wheel. Several can be seen up close on back dirt roads and one—Apishapa Arch—on the south side of the peaks can actually be driven
The dikes are a prominent feature around the Spanish Peaks. There are two systems of radial dikes and a separate, older system of semi-parallel dikes in the area. One of the systems of radial dikes is centered on the West Spanish Peak. The other is centered on Silver Mountain.