08/06/2021
I will always give a big shout out to Oceans.. Past & Present!
It's so let's give a special shout-out to oceans past—strap in for some stratigraphic schooling! 🤓
The famous hoodoos at Bryce Canyon National Park are carved of Claron Formation Limestone, the Pink Cliffs at the zenith of the Grand Staircase. (See the topmost layer at Bristlecone Point, in the attached photo). These are the youngest rocks in the park, which makes sense as they are higher stratigraphically (on top of other rock layers).
When it comes to sedimentary rocks, geologists tend to start from the bottom up, since the older rocks layers lay beneath the younger rocks. 🧱 The oldest bedrocks exposed within the park are known collectively as the Gray Cliffs of the Grand Staircase and, except for the slightly younger Wahweap Formation, all were deposited in the early Cretaceous when much of the continent was covered by an inland sea known as the Western Interior Seaway. 🌊
From the base of the Gray Cliffs in Bryce Canyon National Park, the Naturita Formation (also commonly known as the Dakota Sandstone for its similarity to rocks found on the other side of the seaway, in Nebraska) formed from coastal plains and offshore deposits as the seaway first began to dominate the region. 🏖️ These sandy layers tend to be capped by a thin layer of shell debris 🐚 that spread through the region as the seaway encroached further onto land; this fossiliferous transition marks the lower extent of the rock formation directly above it, the Tropic Shale.
Named in 1931 for the town of Tropic, UT (where this photo was taken 📸), the Tropic Shale was deposited as very fine-grained ocean sediments when this area was submerged fully beneath the inland sea. 🌅 Beneath the pink Claron cliffs, the smooth gray hills in the foreground of this photo are made up of the Tropic Shale. This formation is rich with ammonite fossils , and 🦪 oysters, 🐌 gastropods, 🦈 shark teeth, 🐢 turtles, 🦎 marine reptiles, and 🦖 dinosaurs have all been found within it throughout its extent. (If you're familiar with geology a little further to the northeast, you may also know it as the Tununk Member of the Mancos Shale.)
As a rock, shale is one of the least durable and tends to erode into slopes beneath the more resistant cliffs above. Though clay has the potential to absorb large amounts of water, the orientation of the fine, flat clay flakes means that rain 🌧️ tends to roll over the soil without being absorbed. Coupled with the relative lack of organic material, this tends to make it inhospitable to plants (note how sparse the vegetation is on the slopes below the treeline). In the average year, seeds lay dormant in the dust; however, these shale barrens can erupt into brilliant superblooms following wet winters (so, not this year 😞) when enough water soaks into the soil. Something to watch for! 🌻
Up above the shaley gray slopes and below the pink cliffs there are some more resilient yellowish sandstone outcrops, the Straight Cliffs Formation. Deposited above the Tropic Shale in a delta as the Western Seaway withdrew, the Straight Cliffs show up in larger outcrops throughout the park and the Grand Staircase-Escalante National Monument, but faulting in this location makes the formation appear thinner than it actually is.
From about 100 million to 70 million years ago, much of the continent was covered by a shallow ocean and we see that today by the layers of sediments and fossils preserved therein. Though situated on a high plateau that is marked by erosion today, Bryce Canyon's watery beginnings are told in the rocks deposited here long ago. 🌎
Photo NPS/Cass Hennings
(ch)