On the west flank of the Sangre de Cristo Range, east of Villa Grove, a prominent iron-mineralized area can be seen. Here the ghost mine of Orient marks the site where iron ores were mined in the early days of the Colorado Fuel and Iron Company. Nearby, an abrupt terrace along the edge of the valley marks the position of a fault. Recent gravels are involved in this fault, indicating that movement has taken place here within the last few hundred years. A number of hot springs occur along the base of the mountains nearby. The Spanish Peaks, not structurally related to the Sangre de Cristos, are visible from La Veta Pass highway. These two peaks represent a pair of Cenozoic volcanoes, now deeply eroded and much reduced from their former height. Numerous dikes radiating from the bases of these peaks represent fissures which were filled with lava as the peaks formed. The Great Sand Dunes, close to the Sangre de Cristo Mountains north of Sierra Blanca, are discussed in Chapter II in the section on the Quaternary Period. [Illustration: Spanish Peaks, south of Colorado Springs and southwest of Walsenburg, are twin mountains of volcanic and intrusive rock, the roots of Tertiary volcanoes greatly worn down and reshaped by erosion. This view looks southeast from near La Veta Pass, on U.S. Highway 160. (Jack Rathbone photo)] Park Range and Rabbit Ears Range Bordering the western side of North, Middle, and South Parks, another long north-south trending ridge extends from the Wyoming border toward the center of Colorado. The northern part of this ridge, forming the western boundary of the main mountain mass in the state, is called the Park Range. The structure of the Park Range is similar to that of the Front Range: a huge linear corrugation in the earth’s crust, bounded by faults. Because this area has fewer resistant sedimentary rock layers above the Precambrian basement rocks, it is not prominently edged with upturned sedimentary layers. [Illustration: Hahn’s Peak, a highly eroded laccolith of rhyolite porphyry, lies on the west side of the Park Range, along the eastern margin of the Plateau Province. Placer gold was discovered here in 1865, but the bedrock source of the gold was never found. (Jack Rathbone photo) A geologic section shows the structure of the area.] TERTIARY RED BEDS JURASSIC DAKOTA MANCOS DAKOTA Hahn’s Peak PORPHYRY MANCOS DAKOTA PORPHYRY JURASSIC RED BEDS RE-CAMBRIAN [Illustration: Hahn’s Peak] The range is crossed by Rabbit Ears Pass in the north; Gore Pass near Kremmling marks its southern end. Mt. Zirkel (12,180 feet) and Flattop Mountain (12,118 feet) are the two high points of the range; these and a number of unnamed peaks over 11,000 feet high are upward-faulted blocks of Precambrian granite.