In southern Colorado, the Sangre de Cristo and Wet Mountains were also formed as upthrust blocks. Between them, the Huerfano Basin and adjoining Raton Basin received particularly rapid alluvial deposition. In the Raton Basin, quantities of vegetation were deposited in swamps and marshes, forming the thick coal beds which can now be seen in road cuts west of Trinidad and along the Raton Pass highway. Huerfano Basin deposits contain some of the earliest known horse remains, skeletons of a tiny four-toed horse called _Hyracotherium_ (formerly known as _Eohippus_). [Illustration: Bones of _Hyracotherium_, the “dawn horse,” have been found northwest of Walsenburg in Early Tertiary sediments of the Huerfano Basin. (C. R. Knight painting, courtesy American Museum of Natural History)] Other rising ranges provided material for alluvial deposition in North Park, Middle Park, South Park, and the San Luis Valley. Layers of basalt and volcanic peaks show that as the mountains rose, the crust cracked and allowed lava to rise to the surface in great quantities. Tertiary basalts are very much part of the Colorado landscape: some can be seen west of Granby, others in Table Mountains east of Golden. Near Boulder, Valmont Dike was intruded, though lava may not have reached the surface in that area. Spanish Peaks in southern Colorado, Mesa de Maya, the Rabbit Ears Range, Grand and Battlement Mesas, and many other volcanic features were formed at this time. [Illustration: The town of Golden nestles between the Front Range and South Table Mountain. Tertiary basalt capping South Table Mountain covers beds of the Denver Formation. It thins to the right, or south, indicating that its source was probably to the north or northwest. Buildings in the right foreground are the Colorado School of Mines. (Jack Rathbone photo)] [Illustration: A series of almost vertical dikes radiate from West Spanish Peak. Surrounding sediments are Tertiary. Weathering and erosion along sets of joints in the largest dike have shaped it into the “Devil’s Staircase.” (Jack Rathbone photo)] Most of the rich mineral deposits of Colorado are thought also to have been formed during the early part of the Tertiary Period. Solutions rich in gold, silver, zinc, lead, copper, and sulfides of iron seeped into joints and faults in the crust as the mountains were pushed upward. Ore minerals crystallized out, sometimes in veins in the ancient Precambrian igneous and metamorphic rocks, sometimes in Paleozoic sediments. These are further discussed in Chapter III. [Illustration: The Eocene Green River Formation includes great thicknesses of oil shale, an untapped petroleum reserve containing perhaps three trillion barrels of oil. The richest part of the oil shale is a dark brown layer called Mahogany Ledge, visible here on cliffs just west of Rifle. If placed in a campfire, fragments of this shale release enough oil to burn with a yellow, smoky flame. (Jack Rathbone photo)]