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Raven Ridge Colorado
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Raven Ridge, Colorado

January 14, 2009
An important way to unravel the Earth's history is to find and study old rocks that have been turned up and exposed on the surface through the Earth's tectonic activity. This astronaut photo of Raven Ridge, Colorado, provides a beautiful example of such a place, one that allows geologists to walk across rocks formed about 65 million years ago, a period now known as the boundary between the Cretaceous and Tertiary Periods. The ridge is a dramatic topographic feature in northwestern Colorado formed by layered sedimentary rocks that span this boundary in geologic time. These rocks, originally deposited in a near-shore or marine environment as flat layers, were later tilted on end to an almost vertical position by tectonic forces. The tilted beds are visible in this photo as hard, erosion-resistant ridges of tan, buff, and white rocks, with a softer, gray layer in the center of Raven Ridge (extending from image left to image right).

The Cretaceous-Tertiary (commonly abbreviated as “K-T”) Boundary is most famously known as the geological threshold where dinosaurs—and a large number of other animal and plant species, both terrestrial and marine—disappeared from the fossil record in a mass extinction event 66-65 million years ago. Various hypotheses have been advanced to explain the mass extinction event, the best known being a large meteor impact that sparked widespread climate change. Another hypothesis is that widespread volcanism produced significant climate change that was unfavorable for the existing plants and animals.

The approximate location of the K-T Boundary is depicted in this image as a dotted white line. Rock layers to the south of the line belong to the Tertiary Period (lower half of image), while rocks to the north of the line are part of the Cretaceous and older Periods (upper half of image). A prominent topographic break in the ridgeline, Mormon Gap, provides road access across the ridge. To the northwest of the Gap, several landslides formed in relatively soft Tertiary clay, shale, and sandstone extend southwards from the crest of the ridge.


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