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. At Raven Ridge (image center, left to
right), which extends across the Colorado-Utah border, geologists can
walk across successive layers of rock deposited approximately 54-50
million years ago during the Eocene Epoch—approximately 10–15 million
years after the dinosaurs went extinct.
The sedimentary layers at Raven Ridge were originally
deposited as flat-lying beds in an ancient lake known as Paleolake
Uinta, and later they were tilted by tectonic forces. The tilted beds
are visible in this astronaut photograph as hard, erosion-resistant
ridges of tan, buff, and white rocks (freshwater limestones), with less
resistant layers (mudstones and shales) forming valleys between ridges.
A prominent topographic break in the ridgeline, Mormon Gap, provides
road access across the ridge. To the northwest of the gap, dark brown
alluvial deposits (sand and gravel carried by moving water) extend
southward from the crest of the ridge. These deposits likely cover a pediment—a flat, sloping surface of eroded bedrock that extends outward from a mountain front.
The rocks and fossils at Raven Ridge indicate that during the time
when they were created, the area was covered in tropical forests that
were filled with the early ancestors of modern primates, horses, and
deer (among other animals).
Astronaut photograph ISS018-E-11127
was acquired on December 6, 2008, with a Nikon D2Xs digital camera
fitted with an 800 mm lens, and is provided by the ISS Crew Earth
Observations experiment and Image Science & Analysis Laboratory,
Johnson Space Center. The image was taken by the Expedition 18 crew. The image in this article has been cropped and enhanced to improve contrast. Lens artifacts have been removed. The International Space Station Program
supports the laboratory to help astronauts take pictures of Earth that
will be of the greatest value to scientists and the public, and to make
those images freely available on the Internet. Additional images taken
by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by William L. Stefanov, NASA-JSC, and Alexander R. Dutchak, University of Colorado, Boulder, Colorado.