Online guide to the continental Cretaceous-Tertiary boundary in the Raton basin, Colorado and New Mexico

Description of the Route from Denver to Raton

Castle Rock to Colorado Springs

The Castle Rock itself and many of the mesas south of the town of Castle Rock are capped by the Castle Rock Conglomerate. In this area, except at Castle Rock, the conglomerate is locally underlain by the silicic Wall Mountain Tuff. The Wall Mountain is an ash flow tuff that occurs on the Rampart Range to the west and is also present in the Thirty-nine Mile volcanic field. The tuff was a widespread ash flow that erupted 34.8 ± 1.1 Ma (J.D. Obradovich, USGS, written commun., 1969) from its presumed source near Mt. Aetna in the Sawatch Range, about 100 mi. to the west.

The route continues in the Dawson Arkose nearly to Colorado Springs. In the vicinity of the Air Force Academy, outstanding examples of pediments of three ages can be seen west of the highway, capped from highest to lowest by the Rocky Flats, Verdos, and Slocum Alluviums. Precambrian rocks of the Rampart Range, which lies west of the pediments, are uplifted about 10,000 ft against Dawson Arkose by the Rampart Range reverse fault. This fault, the Ute Pass fault to the south, and other faults along the Front Range may be low angle thrusts, on the basis of geophysics and the outcrop pattern (A.F. Jacob, consulting geologist, written commun., 1985,). The large scars visible on the mountain front of the Rampart Range north of Colorado Springs are quarries that were developed for concrete aggregate in limestone and dolomite of the Devonian Williams Canyon and Ordovician Manitou Limestone Formations.

Just past the south entrance to the Air Force Academy, I-25 crosses Monument Creek and passes down section through outcrops of the coal bearing Laramie Formation and the Fox Hills Sandstone (mostly covered), which is lithologically equivalent to the Upper Cretaceous Vermejo Formation and Trinidad Sandstone of the Raton basin, (Fig. 2) and into the Pierre Shale, all of Late Cretaceous age. Much of the area immediately east of Monument Creek is the site of abandoned coal mines in the Laramie. Continuing south through Colorado Springs, good views are afforded of Pikes Peak, which is underlain by the Precambrian Pikes Peak Granite (about 1,000 Ma), and Cheyenne Mountain, underlain by granodiorite of the Routt Plutonic Suite (about 1,700 Ma; Tweto, 1987). Cheyenne Mountain houses the large underground installation of NORAD (North American Air Defense Command). The Ute Pass fault, a large reverse and thrust fault that dips to the west as low as 30 degrees and trends along the eastern base of the mountain, places Pikes Peak Granite against Upper Cretaceous Pierre Shale. To the west of Colorado Springs, the Fountain Formation is spectacularly displayed in erosional forms at the Garden of the Gods. These arkosic sandstone and conglomerate units are about 4,000 ft thick and were deposited as alluvial fans on the east flank of the northwest-trending ancestral Rockies during Pennsylvanian and Early Permian time.

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