The Ancestral Rockies were an ancient Paleozoic mountain range that existed in western North America in the location of the present-day southern Rocky Mountains in Colorado. The range was formed approximately 300 MYA during the Pennsylvanian by an uplift process known as the Colorado orogeny, itself a result of a period of intense sea-floor spreading. At the time of the uplift, western North America in the area of present-day Colorado was largely covered by a shallow sea. The uplift created two large mountainous islands, known to geologists as Frontrangia and Uncompahgria, located roughly in the current locations of the Front Range and the San Juan Mountains. They consisted largely of Precambrian metamorphic rock forced upward through layers of limestone laid down in the shallow sea throughout the early and late Paleozoic.

The mountains eroded throughout the late Paleozoic and early Mesozoic, leaving a large deposition of debris in the surrounding floodplains that formed layers of sedimentary rock. By Jurassic time, the mountains had completely eroded away. The current Rocky Mountains were subsequently created by a second uplift, the Laramide orogeny, that occurred in roughly the same location as the Ancestral Rockies and which began in Cretaceous time approximately 70 MYA. By the time of the second uplift that created the current range, the original mountains had long since eroded away. The current Rockies were forced upwards through the layers of Pennsylvanian and Permian sedimenatry remnants of the Ancetral Range. Such sedimentary remants were often tilted at steep angles along the flanks of the modern range and visible in many places throughout the Rockies, including prominently along the Dakota Hogback, a Pennsylvanian sandstone formation that runs along the eastern flank of the modern Rockies.

External link

• Southern Rockies Geology

References

• Roadside Geology of Colorado, Halka Chronic (1980).

History of Colorado | Rocky Mountains