(9/22) Geology of Capitol Reef
The second day, we followed a west-to-east route across the park with the aid of a book that identified the rock formations The rocks exposed along this route span the Triassic and most of the Jurassic periods -- the age of dinosaurs -- and they exhibit considerable variety.
What we learned is that, here at least, the thin-bedded, easily eroded rocks (shales, siltstones, and some sandstones) were deposited in wet environments, while the thick erosion-resistant sandstones were formed in desert conditions by wind blown sand.
The oldest formation we encountered was the Moenkopi, chocolate-brown shaley slopes at the bottom and columns of alternating siltstones and sandstones above. A shallow sea here fluctuated in level, so that sometimes this area was near the shore where the water had considerable movement and at other times this area was farther offshore where there was little or no current. Of the particles carried down to the sea from the mountains, the sand dropped out first, near the shore, while the silt got carried into deeper water and the finest particles (forming shale) were deposited farthest from shore.
Above the Moenkopi is a striking gray-green band of soft claystone, part of the Chinle formation, which was deposited by rivers after the sea had retreated to the west. Other parts of the Chinle continue up to the base of the high cliff, including some thin-bedded sandstone that was deposited by fast-flowing rivers.
The high cliff itself is Wingate Sandstone, formed from quartz sand that was deposited when this area was part of a huge desert.
Here is a closeup of the Kayenta formation, from the early Jurassic, with soft rocks from intermittent stream flows (thin-bedded) sandwiched between hard desert sandstones from dryer periods (thick-bedded).
Above the Kayenta is another massive desert sandstone, the Navajo. The Sahara-like desert that existed in the West at that time was one of the largest ever known. The whitish rock of the Navajo Sandstone tends to weather into rounded forms such as the one in this picture, called Capitol Dome.
Above the Navajo is the reddish Entrada formation, When we saw it in Arches National Park, it was the massive desert sandstone from which the arches were formed. Here the desert sandstone alternates with siltstone layers deposited under tidal-flat conditions, indicating that this area was near the edge of the dune field.
We saw several younger layers as well, some resembling the older layers, but you get the picture. The rocks are beautiful on an esthetic level and part of the beauty is in their variety. We found it satisfying to know the reasons for the variety.
We should mention that we had two cloudless nights here, outside the small town of Torrey, where there is no sky glow from city lights. The B and B where we stayed was on a high ridge with 360-degree views. The Milky Way was clearly visible and there were so many stars that it was hard to make out constellations.
