I love plants and am a native plant master in Colorado. I like to say that plants don’t fly or run away like birds and animals, and their life spans are understandable. Geologic time and processes have been more elusive for my mind to wrap around. Yet, in planning to guide a recent trip to Utah’s national parks, I was inspired to better understand geology — just thinking about the huge variety of colors, shapes, and textures of rock sparked my curiosity. And so to better understand geology, I turned to sketching, while I studied.
The Colorado Plateau is a great place to learn geology because the many layers of sedimentary rock can be seen throughout the area. The story of the area basically has 4 chapters:
Chapter 1 Deposition: Over millions of years, seas and sand dunes formed, receded, formed again, and again, creating distinct layers of sediment.
Chapter 2 Lithification: Rocks form in underlying sediment layers by pressure from overlaying layers and minerals that cement the sediment particles. Some minerals are like “super glue” creating super hard rock, while others are like “Elmer’s glue” creating softer rock that is more susceptible to water in later chapters. Geologists have aged and named the distinct rock layers. (See chart below)
Chapter 3 Uplift: Tectonic forces in the earth uplifted the Colorado Plateau about 65 million years ago (more or less), creating stresses in some areas that enabled chapter 4 to do its work in fantastic ways.
Chapter 4 Erosion: The powerful force of water is the most important character in this chapter. Flood waters carry rock, trees, and other materials that scour and sculpt the mesas, canyons, and valleys into the incredible beauty we see on our trip.
Geologic time is tough for me to comprehend. But like learning anything, baby steps are in order. To get a better understanding of the sedimentary layers and geologic time in the parks on our itinerary, I made a chart, which brought home both how similar and different the four parks are. Uplift and geologic processes created some mysteries in the layers and messiness on my chart — I had to white-out a bit!
Starting at Zion National Park, I learned that the canyon is wide at the Springdale entrance and becomes narrower and narrower toward the Narrows. This is because the Virgin River has eroded and cut downward for millions of years. The Navajo sandstone’s quartz crystals are cemented so hard that the river can only cut narrow channels. But the elevation change creates more force as the the river flows down canyon, cutting into the underlying (older and softer) Kayenta and Moenave sandstone and siltstone, which erodes more easily. This process undercuts the Navajo sandstone. Then, over time, gravity causes the majestic Navajo sandstone walls to fall, widening the canyon.
Also, those majestic sandstone walls are basically petrified sand dunes from the Juassic Period, about 200 million years ago. It’s hard to imagine huge sand dunes here.
Bryce Canyon was our second stop. The rock hoodoos are formed in rows because when the Colorado Plateau uplifted here, the sedimentary layers ‘cracked’ in vertical and horizontal rows, enabling water to seep into the rock layers. Freeze/thaw cycles over millions of years caused erosion and the softer layers eroded more quickly, while harder layers on the caps and interior of the hoodoos bulged or resisted erosion.
The Paiute people lived in this area and the NPS reports a Paiute elder telling a park ranger that “before there were humans The Legend People, To-when-an-ung-wa, lived in that place- birds, animals, lizards and such things. They had the power to make themselves look like people, but they were not people. They did something bad, it is unclear what, and so coyote turned them all into rocks. You can see them in that place now; some standing in rows, some sitting down, some holding onto others. You can see their faces, with paint on them just as they were before they became rocks. The name of that place is Angka-ku-was-a-wits (red painted faces). This is the story people tell.”
Tragically, the Paiute lost their homelands when nonnative immigrants took over this area. (It reminds me of the all of the Eurasian invasive weeds who have crowded out and eliminated the native plants in the openspace near my home).
Capital Reef National Park is park number three on our itinerary. The park was named for large sandstone domes that look like the US Capital building, and for the 100-mile Waterpocket Fold which was impassible for 19th century pioneers. This is an area where the sedimentary layers were uplifted about 65 million years ago in a way that one area folded over the other, also creating numerous depressions which fill with water, creating “waterpockets.”
Arches national Park is the 4th and last stop on our trip. Unfortunately we didn’t have time to see all 2,000 arches in the park this time, but the Delicate Arch definitely lived up to its reputation. This area is perfect for arch formation because the wavey Dewey Bridge member lies under the Slickrock member of the Entrada Formation. When the plateau uplifted, parallel joints formed, which has allowed water to seep down and settle in the spaces between those two layers of sandstone. Over time, ‘fins’ formed and then the softer rock along the juncture eroded in arch shaped. Water and gravity keeps widening the arches, until they eventually fall.
In my next post I’ll share some of the flowering plants I sketched along the way!
Brenda Claire Porter 