The Great Dikes were formed during the same period of volcanic activity as the Spanish Peaks, Mt. Mestas and Silver Mountain. At the time these vertical granite formations were formed by molten rock, they were located several thousand feet underground, below and among many layers of sedimentary rock. Over time, as the ground rose and the softer rock was eroded away, these igneous intrusions were exposed.

There are essentially three different sets of dikes in the area. One set emanates radially from the West Spanish Peak. The second set emanates radially from Silver Mountain. The third set crosses the landscape in a roughly N80E direction. The dikes in this third set are roughly parallel to one another and are the longest and oldest of the dikes. This third set of dikes was formed about the same time as the Sangre de Cristo Uplift, the event that pushed up the Sangre de Cristo Mountains between 2 north-south running fault lines about 27 million years ago. The Spanish Peaks, Silver Mountain, Mt. Mestas, the White Sisters, the Sheep Mountains, Iron Mountain, and the Black Hills in Huerfano County, along with their associated radial dike systems, were formed about 25 million years ago, east of the Alvarado Fault (which defines the east side of the Sangre de Cristo's) and east of the Dakota Wall. The volcanic plugs at Gardner, Huerfano Butte and Goemmer Butte were formed at the same time.

The dikes are granite. In the western portions of both Las Animas and Huerfano Counties there are some uplifted stone walls of the Dakota formation. They look like granite but they are actually very compressed, very durable sandstone, part of a formation that runs from Canada to Mexico along the Front Range and eastwards. Other places where it surfaces vertically are in the Flatirons area of Boulder, Garden of the Gods in Colorado Springs and the Stoneridge between Cerrillos and Santa Fe, New Mexico.