Monday, August 4, 2008
Time Beyond Imagining - A Brief History of the Colorado Plateau: Sand!
Back to our adventure through time on the Colorado Plateau. We have been mostly exploring the Proterozoic and Paleozoic history with a walk up from the bottom of the Grand Canyon where these rocks are so spectacularly exposed. There have been some side excursions to the Paradox Basin and the Ancestral Rockies, but today we reach the rim of the canyon, with a short exploration of the final three formations that make up the canyon walls (in two posts).
One of the most obvious layers in the Grand Canyon is a 300 foot high white cliff just a few hundred feet below the rim. The rock making up the cliff is the Coconino Sandstone, which formed in Permian time as sand dunes in a desert that extended across northern Arizona, New Mexico, and southern Utah (something like 200,000 square miles). The most obvious feature of the formation are the crossbeds, the sloping layers that represent the slip faces of the dunes that have been preserved in the cliffs. It is a beautiful rock formation!
When I was a new student taking my first or second class in geology, I had the opportunity to visit the Raymond Alf Paleontology Museum at Webb School in Claremont, California. I was intrigued by their spectacular collection of fossil trackways, and was curious where such ichnofossils were found. One of the sources was the Coconino Sandstone, which I would see up close only a few weeks later on my first geology field trip. There is nothing quite like finding a fossil trackway like the one illustrated above. Unlike fossil shells and bones, representing death, a trackway preserves a moment in the life of a creature. Was it running or walking, feeding or escaping? What kind of creature was it anyway? The Coconino preserves more than two dozen kinds of tracks, from large amphibians or reptiles to scorpions or spiders.
The Coconino is correlated with the De Chelly Sandstone, which forms the spectacular cliffs of De Chelly National Monument, and the wonderful John Wayne movie towers and spires of Monument Valley. Because the sand is porous, it is an important groundwater reservoir for towns to the south of Grand Canyon like Flagstaff and Williams.