Friday, June 21, 2019

Super Bloom on Tropic Shale

Looking up the Grand Staircase from the Kaibab Plateau (photo by Patti Weeks; source).
Like any staircase, the Grand Staircase of Grand Staircase-Escalante National Monument in south central Utah is composed of risers and treads. The risers are sandstone cliffs of various colors—chocolate, vermilion, white, gray and pink—that ascend from the Grand Canyon north to Bryce Canyon. Between the risers are the treads—flat to sloping terrain developed on softer more easily-eroded rocks. Though the treads don’t get nearly as much attention as the spectacular long cliff bands, they're interesting too, especially this year.
A layer cake of sedimentary rocks underlies the Grand Staircase. Modified from Doelling et al. 2000.
The tread of interest is the Tropic Shale.
Above the Gray Cliffs (Dakota sandstone) and below an unnamed riser made from the Straight Cliffs Formation, is a tread made of the Tropic Shale. This shale started as fine sediments on the bottom of the Western Interior Seaway, which covered much of central North America roughly 90 million years ago (late Cretaceous time). “Tropic” has nothing to do with the environment of deposition, instead referring to the town of Tropic, Utah, just west of the National Monument, where there are excellent exposures of the shale. This has confused some rather prominent botanists, including those working with the Federal Interagency Taxonomic Information System (more below).

Being relatively soft, the Tropic Shale erodes to form slopes and flats below sandstone cliffs of the Straight Cliffs Formation. The soil that develops is fine-textured, like the ocean-floor sediments it once was. It’s inhospitable to plants because of the way those fine particles interact with water (1). Normally, the little rain that falls isn’t absorbed into the soil, and runs off instead. But in wet years, enough soaks in for plants to grow. If it’s really wet, like this year, they go gangbusters!
The Tropic Shale and Straight Cliffs Formation are easy to spot along the Cottonwood Canyons Road, a popular geology tour (when dry). The photo above was taken near the junction with road 431, just a mile and a half north of Highway 89. The bluish-gray flats and low slopes, and the darker slopes above, are made from the Tropic Shale. The cliffs at the top are sandstone—part of the Straight Cliffs Formation, which was deposited after the Tropic Shale in a delta west of the retreating Western Interior Seaway.
But what about those large yellow patches on the flats? I was pretty sure I knew what they were, as I had recently read about shale barrens in Spring Wildflowers of Utah’s Red Rock Desert (Lesica and Fertig 2017):
“Shale barrens occur on slopes and alluvial plains where dense, shale-derived clay is at the surface. There is little or no organic matter in the soil, and water runs off rather than percolating downward. … There is little or no vegetation in most years, but spectacular blooms of annuals … may occur following a wet winter.”
This winter was indeed wet, as was spring. “Must be flowers!” I thought, and started walking.
Getting closer …
I was right. In fact, it was a super bloom! Among the hordes, I saw only two species: Colorado Plateau Stinkweed (Cleomella palmeriana) and Tropic Goldeneye (Heliomeris soliceps, formerly Viguiera soliceps). Both were easily identified with the Spring Wildflowers guidebook (highly recommended, see Sources below).
A shale barren super bloom.
Colorado Plateau Stinkweed (above and below) is endemic to the Colorado Plateau. According to Flora of North America, it grows on “dry open alkaline, gravelly or sandy flats.” Lesica and Fertig report it from “barren, clay or silty soils in salt desert shrub and clay barren habitats.” They note that it “can turn gray clay barrens yellow for a few weeks following a wet winter, but then will be essentially absent in dry years.”
Tropic Goldeneye (below) is even more limited in distribution. Flora of North America reports that it grows on gumbo clay knolls and bluffs in Utah, and is of conservation concern. The common name is said to be Tropical Goldeneye, but we enlightened geo-types are sure this must be incorrect! Surely it is named for the Tropic Shale—and Lesica and Fertig confirm that this is the case. The Tropic Goldeneye grows on “barren, clay soil derived from the Tropic Shale in salt desert shrub communities. Known only from south-central Utah. … it is abundant where it does occur in years with good winter rainfall.”

Some readers, having been hooked by the title, may be disappointed by my super bloom. After all, the flowers are all the same color, and aren't continuous. Just look at all that bare ground! But it was indeed super—super wonderful to be in a wild landscape with thousands of blooming plants that most years are just seeds lying in the dust of a shale barren, waiting for a deluge.
No selfies from this super bloom, just a shot of my field assistant (center left; click image to view).


(1) Fine-textured soils are often inhospitable to plants because of the physical characteristics, especially in environments with little rainfall:
“In arid ecosystems, fine-textured soils are noted for absorbing and storing water less efficiently than coarse-textured soils … Due to the large surface area to volume ratio and flake-like structure, clay particles may absorb a great deal of water; however, due in part to the layered orientation of the particles, much of the water is unavailable for plant growth. Clay soils have very small pore spaces and retain water against the power of roots to extract it …” (Silva & Ayers 2016).

Doelling, HH, et al. 2000. Geology of Grand Staircase-Escalante National Monument, Utah, in Sprinkel, DA, et. al., eds. Geology of Utah’s Parks and Monuments. Utah Geological Association Publication 28.

Lesica, P, and Fertig, W. 2017. Spring wildflowers of Utah’s Red Rock Desert. Missoula: Mountain Press.
Highly recommended: Because I don’t know the local flora beyond genus, I was able put this book to the test. It was easy to use and informative. It contains the 300 most common plants of the region, and included all but one species that I tried to identify. More here.

Share, Jack. January 28, 2011. The Grand Staircase section of the Grand Staircase-Escalante National Monument, in Written in Stone … seen through my lens

Silva, S, and Ayers, T. 2016. Plant endemism on Mancos shale barrens. Natural Areas Journal 36:166-173.


  1. Enjoyed this one Hollis! I really need to make a trip to the west and south west of the U.S. :)

    1. Thanks, Suvrat. Yes, so much wonderful country there!

  2. For those who noticed: Sorry for the poor diagram of the Grand Staircase sedimentary layer cake--just replaced it with one that enlarges nicely. The duplicate photo was Blogger's fault, I have no idea why. When I updated, it behaved itself.

  3. It's been so, so long since I've been to Utah. It's definitely on the return bucket list. Thanks for the info on the Grand Staircase. The super bloom is delightful!

    1. Beth, I hope you get to visit GSENM--so big, so diverse, and still not overrun with visitors!