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].
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.
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
