Still decompressing after a "long" trip of two weeks through geologic time on the Colorado Plateau. It is always difficult to comprehend the contrast between the hours and days of one's human existence with the millions and billions of years that characterize the geologic history of our planet and Universe. I expect to follow this theme in my blogs for a few days as I sift through the pictures of our journey.
The unusual view above comes courtesy of astronomical lunar cycles and Landscape Arch in Arches National Park. The arch has an opening nearly as long as a football field, and is only 6 feet or so across at the thinnest point. It is rather a wonder that the span still stands, and the future prognosis of the arch is probably measureable in human, not geologic, terms. A large chunk fell from the span around 1991.
The arches in the park are formed in the Slickrock Member of the Jurassic Entrada Sandstone. The unit has been pushed upwards by bodies of mobile salt from the late Paleozoic Paradox Basin. The upward pressure in the salt anticline has fractured and jointed the sandstone into a series of "fins", long narrow walls of rock. The base of the fins lie in the Dewey Bridge member of the Entrada (or Carmel), which is easily eroded, allowing a small window to form at the base of the fin. Chunks of rock fall from the window, until an arch develops. There are hundreds in the park.