OF-17-06 Geologic Map of the Boreas Pass Quadrangle, Park and Summit Counties, Colorado


Extending the CGS mapping efforts in the central portion of the state, this publication describes the geology of this 7.5-minute quadrangle located just to the east of Breckenridge, Colorado. Digital ZIP download. OF-17-06D

From the Geologic Overview:

The Boreas Pass quadrangle, located entirely above elevation 9,800 ft (2987 m), is in the far southwestern part of the Front Range, a major constituent of the southern Rocky Mountains. A small area in the southeastern map area is in the northwestern part of South Park, a large, structurally complex intermontane basin. Geologically, the quadrangle is very complex, having experienced several periods of structural deformation and uplift, igneous intrusive activity, hydrothermal alteration and mineralization, and intense alpine glaciation.

During Quaternary time, glaciers produced numerous cirques and the deeply glaciated valleys of French Gulch and the Middle and South Forks of the Swan River on the northwest side of the Continental Divide, and Michigan Creek, French Creek, Jefferson Creek, and North Tarryall Creek southeast of the Divide. Deposits of glacial till and outwash were produced. Rock glaciers are present and some may be active in several of the high cirque basins. A few visually spectacular protalus rampart deposits were formed from loose rocks falling and being deposited as arcuate ridges along the bases of long-lasting snow and ice fields. Rich placer gold deposits formed during the Pleistocene mainly in glaciofluvial outwash, especially along French Gulch in the northwestern part of the map area. Placers in the non-glaciated, smaller gulches north and east of Farncomb Hill were formed by mass-wasting and erosion of supergene deposits. Landslides cover a substantial portion of the Boreas Pass quadrangle and are the most extensive geological hazard. Other geologic hazards in the map area include rockfall, debris flows, floods, and swelling soils. No evidence of Quaternary faulting was found in the map area.