Travel opportunities to far-away geologically exotic destinations do not seem to be looming on my personal adventure horizon this week. Luckily, I happen to live in a place where people come from all over the world to experience the incredibly scenic red-rock landscape; more often than not I don’t even have to go very far to be watching for various lovely rocks. My “travels” on this particular day trip involved tossing my camera, a windbreaker and a bottle of water into a backpack, shifting my car into gear, driving 20 miles to get out of my neighborhood in the snow, and then meeting a friend at the Chuckwalla trailhead parking lot in the Red Cliffs Desert Reserve near St. George, Utah.
Tilted sandstone beds appear to be ledgy cliffs of upper Kayenta Formation
The Reserve is a hiker’s and geologist’s paradise but there were only a few other people out and about on this balmy 42°F brilliant-blue-sky early December day. For years I have wondered about these rocks – are they the top of the Kayenta Formation? Or are they the base of the overlying Navajo Sandstone? They show up very distinctively and prominently in the cliffs of Zion National Park, barely 50 miles away. But near St. George, where millions of years of plate tectonics have pushed, stretched, and tilted the crust much more so than in the Zion area, the boundary between the Kayenta and Navajo can be less clear. At least to me it can be less clear. And since I am usually willing to exchange confusion for clarity I decided to check out the Chuckwalla.
Approximately 1.0-1.2 million year old basalt flow eroding onto sandstone
Basalt eroding down sandstone cliff;snow-covered Pine Valley Mtn. in distance
My local geologic map shows that the rocks on this particular patch of ground are mainly Early Jurassic (around 190 million years old) Kayenta Formation with assorted more recent volcanic basalt flows plastered across the hillsides. The rocks of the Kayenta are siltstones and sandstones, outcropping as alternating reddish slopes and ledges, and record an environment of shallow lakes and low to moderate-energy streams coming off ancient mountains to the west. Fossilized mud cracks tell of drying conditions. Thin limestone layers and trails of aquatic snails and worms reveal the existence of ancient ponds and lakes. Commonly occurring in the siltstones are tracks of upright-walking three-toed dinosaurs.
This was the time of the break-up of Pangaea, when the supercontinent was centered more or less on the equator. It was a time of seasonal climate with wet summers and cold winters at the edge of a vast desert, the influence of which was soon to predominate as plate tectonics drove prehistoric North America to drift incrementally northward into an arid desert belt. Here the lakes and streams of the Kayenta would eventually give way to the immense dune fields of the future Navajo Sandstone.
Chuckwalla trail winds through upper Kayenta Formation sandstones
Sue ponders rocks and birdlife
Across highway 18 from the trailhead the younger Navajo Sandstone (Early Jurassic Period, about 180 million years ago) outcrops as massive hills. It dawned upon me that the trail most likely cuts through the very upper layers of the Kayenta and possibly the very lower part of the Navajo – the map description states that these ledgy slopes of the Kayenta grade up to ledgy sandstone cliffs at the top of the formation. And that is exactly what I was seeing.
As I moseyed along the trail in between the rocks I contemplated the almost inconceivable changes that had occurred those hundreds of millions of years ago when the earth’s plates were shifting, its climate was changing, and these ancient sediments were being deposited so that, two hundred million years in the future, I could look at these rocks and appreciate the wonder to be found in them.
Probably not the best location to build a house!
Tilted sandstone beds appear to be upper Kayenta Formation