Geology of Slim Buttes II—a literary perspective

[For geological detachment and unconformity at Slim Buttes, see previous post.]

The Mystery of Slim Buttes

by Danny Rosen

The mystery of Slim Buttes is that,

laid bare, ‘ol Slim, as he was known,

was slimmer than ever. No slouch, but

he slipped, above and below didn’t add up.

Detached, Slim slid off to love only

his dog - who loved to hunt

Reva’s Gap and figure mysteries

with his snout. Slim learned history

and held dear to his doubt.

An angular guy who tried as he might

could not conform, so he remained

close to his castle, true to his form.

Danny Rosen is a geologist, poet, owner of Lithic Press, and major supporter of the literary scene in Fruita, Colorado. You can find him most days at his bookstore and gallery.

The Puzzling Geology of Northern Slim Buttes

Slip slidin' away?

The Slim Buttes of northwest South Dakota are not your typical textbook buttes—not isolated hills with steep sides and flat tops. To me they look like a single ridge, or a "long and narrow tableland" (Gries 1993). Perhaps Slim Buttes are what North Dakota geologist John Bluemle (2016) calls "buttes in name only"—landforms that are buttes because that's what they were christened.

No matter; they're noteworthy whatever we call them. The tableland extends about 30 miles north to south, is generally less than a mile wide, and rises as much as 600 feet above the prairie. With ponderosa pine on the upper slopes and crest, the Buttes stand in clear contrast to the grassland below, a rolling green sea that reaches to the horizon in every direction.

In the northern Buttes, in the area of Reva Gap, the land has slid, fractured and tilted, producing the Castles—a mix of white badlands and green grass (in spring). The site was designated a Registered Natural Landmark in 1979 because it "illustrates the Nation's natural heritage" and "contributes to a better understanding of man's environment". Or rather it will once someone figures out what happened!

We do know that the Slim Buttes tableland is a product of erosion—a remnant of the Tertiary sedimentary rocks that used to cover the region. It's capped with hard sandstone of the Arikaree formation, which has protected older rocks below from erosion. Those softer underlying rocks are exposed at Reva Gap.

Castles stratigraphy. Not visible but important to the story is the Ludlow member of the Fort Union formation.

This stratigraphic column spans c. 50 million years, from late Cretaceous (c. 70 my ago) through lower Miocene (c. 20 my ago). The Brule and Chadron belong to the White River Group. The Chadron includes three informal units—typical Chadron, dazzling white beds, and golden beds (top to bottom).

The Castles have features that geologists love—angular unconformities. They are easy to spot, and nicely illustrate important concepts. A geologist can stand next to one and expound on some of the most basic principles of geology: original horizontality, superposition, and cross-cutting relationships. And they provide clues as to how the landscape formed.

Behold an angular unconformity!

This schematic shows the birth of a textbook angular unconformity:

It seems this story would apply to the Castles, where tilted beds of the Brule formation are capped with horizontal beds of the Arikaree formation. The rocks of the Brule, being sedimentary, must have begun as horizontal beds of sediment (principle of original horizontality), but now they're tilted. Therefore they were deformed somehow, maybe during mountain-building. The Arikaree rocks atop the Brule are younger (principle of superposition), and being horizontal, were deposited after deformation of the Brule ceased (principle of cross-cutting relationships).

However, for the Castles this story is incomplete, and could lead us to false conclusions. The final diagram in the schematic above should also include strata below the tilted beds, for herein lies a problem:

Oil and gas exploration has revealed that older sedimentary rocks out-of-sight below the tilted Brule and Chadron are horizontal! This is very hard to explain. How were beds tilted independent of beds below? Furthermore, this deformation is limited to northern Slim Buttes; no tilted Brule and Chadron have been found in the southern part. What a puzzle this jumble of rocks presents! Geologists still struggle to solve it.

John Paul Gries (1911-2003) was on the faculty of the SD School of Mines and Technology for fifty years. He's best known for Roadside Geology of South Dakota; more than 500,000 copies have been sold.

Roadside Geology of South Dakota (Gries 1993) is a wonderful guidebook, because it covers much more than what's visible from roads. For example, three pages are devoted to the Reva Gap area, including a history of geologists trying to explain its geologic history.

They came up with diverse theories that mostly failed: sharp folding and erosion; large-scale cross-bedding; major folding at a regional scale; and tilted blocks slumping off steep cliffs undercut by streams. The list ends with a "recent suggestion"—failure in very weak clay layers causing dropping and tilting of overlying beds. Gries didn't dispute this hypothesis, but concluded that "The field of speculation and interpretation is still wide open."

That recent suggestion appeared in "Stratigraphy, structure, and vertebrate fossils of the Oligocene Brule formation, Slim Buttes" (Lillegraven 1970; "recent" as of 1993). The author noted that faulting and tilting affected all strata from the Brule and Chadron formations down to at least the upper part of the Ludlow member. The Ludlow is key: "There are many shaley layers within the Ludlow or upper Hell Creek beds that could have acted as glide planes." Lillegraven suggested the underlying cause was "minor northeast movement of Ludlow through Brule sediments along a gravity-controlled detachment fault, or series of faults, somewhere deep in the Ludlow."

Detachment faults are low-angle faults along bedding planes that allow overlying strata to slide. This can cause faulting and tilting of the moving strata. Lillegraven theorized that "as the overlying sediments moved along the low-angle glide plane, there was a tendency to break into individual blocks, and superficial normal faulting [and tilting] occurred due to lateral spreading of soft clay from beneath firmer material."

A textbook detachment fault slip slidin' away. Is this what happened at Reva Gap?

Forty-five years after Lillegraven proprosed his detachment hypothesis, Ferguson et al. (2015) announced the presence at Reva Gap of a low-angle detachment in the basal orange and white sandstones of the White River Group [dazzling white beds and golden beds of the Chadron]. High-angle normal faults bounding tilted blocks of Chadron and Brule rocks connected to the detachment. But this can't be the full story. As they noted, underlying Fort Union rocks (Ludlow member) also are tilted. Maybe there's another detachment fault, deeper and out-of-sight. "This idea requires further field research," they concluded.

So the geologic story of the Castles remains unfinished, and the recommendation that Dr. Gries made to Castle visitors back in 1993 still stands: "Poke around and form your own theory." And don't worry if you're not up to the task. Even if no theory presents itself, I bet you will enjoy the experience.

Photogenic Chadron formation; Brule behind.

Yet another mystery to be grateful for.

Sources

Bluemle, JP. 2016. North Dakota's Geological Legacy. North Dakota State University Press.

Diffendal, RF. 2017. Great Plains Geology. University of Nebraska Press.

Ferguson, S, et al. 2015. Synsedimentary low angle normal detachment in White River Group strata of NW South Dakota. Abstracts with Programs, GSA 45 (7).

Gries, JP. 1996. Roadside Geology of South Dakota. Mountain Press Publishing Co.

Lillegraven, JL. 1970. Stratigraphy, structure, and vertebrate fossils of the Oligocene Brule formation, Slim Buttes, northwestern South Dakota. GSA Bull. 81:831-850.

Wildflowers at Slim Buttes

There are flowers in the grass among the Castles in the Gap in the Buttes on the prairie.

In Harding County in northwest South Dakota, roughly 40 miles east of Montana, stand the Slim Buttes—a prominent pine ridge that looks rather out of place on the vast rolling prairie. The Buttes offer much to the curious naturalist. For example, in the northern part at Reva Gap, the land has shifted, tilted, slid, and slumped to create the Castles. Some call this area badlands, meaning an environment so harsh that few plants survive. But that's not the case. Among the Castles are grasslands, and in the grasslands grow prairie wildflowers—my friends of many years.

The Castles are the high points of a curious landscape—a mix of flat to rolling grassy surfaces and very steep barren slopes. Erosion has obviously played a role, but why the flat grassy surfaces? And why do underlying strata tilt discordantly? The puzzling geology of northern Slim Buttes will be the subject of a later post (not promising any answers). What I offer here is much easier to understand—the appeal of wildflowers.

This year Harding County was blessed with a cool wet late spring, after two very dry years. When I visited in early June, the grasslands were still rich green, and it was tricky to photograph wildflowers among such vigorous grasses. Included here are four of the more cooperative species, supplemented with Flickr photos generously provided by Matt Lavin and Patrick Alexander (CC BY-SA 2.0 and Public Domain respectively).

The isolated patch of grassland on the little butte pictured below was filled with Grassy Death Camas, Zigadenus venenosus var. gramineus (1). As both the common and scientific names indicate, this is a poisonous plant. In fact it is one of the most toxic plants of the American West for livestock, and is highly resistant to herbicides. Dried plants may remain toxic for as long as twenty years (USDA Forest Service).

Cream-colored dots on the left half of the flat top are Grassy Death Camas. Cowboys and shepherds needn't worry for their wards here :)

Several accessible individuals; it's hard to see their grass-like basal leaves.

For us, there's no need to avoid Grassy Death Camas (just don't eat the bulbs). We can enjoy the elongate neatly-arranged bouquets without risk. The flowers are trimerous, in keeping with the general rule for monocots (2). Six cream-to-white tepals ("tepal" is used when sepals and petals look alike) and six stamens surround a yellow-green ovary topped with three styles. The grass-like leaves explain the varietal name, gramineus.

Inflorescence with flowers progressively younger from bottom to top. Photo by Matt Lavin (Flickr).

Trimerous flowers of Grassy Death Camas. Photo by Patrick Alexander (Flickr); cropped.

Flowery grassland below another curious Castle; note horizontal layers atop tilted.

Most of the yellow dots in the grassland above are Lambstongue Ragwort, Senecio integerrimus var. integerrimus. It belongs to the Asteraceae (aster, daisy or sunflower family), and like all members, it has many small flowers clustered into tight heads that look like single flowers. This family used to be called Compositae, an apt name given that what at first glance appears to be one flower is actually a composite of many.

In this photo there are several dozen flower heads. Each head contains many flowers.

Lambstongue Ragwort has two types of flowers in each head. In the center is a round cluster of tiny tubular disc flowers. The disc is surrounded by 5–8 ray flowers; these look like "petals" to the uninitiated. (In fact a ray is five fused petals. Go evolution!)

Now you can impress your friends on hikes!

The leaves are said to be shaped like a lamb's tongue—anybody agree?

A beauty hiding in the grass—spiderwort!

Spiderworts were common but fairly well hidden, being shorter than the grasses. Fortunately their flowers are showy, and I arrived early enough to catch them fully open (they close as the day warms). Actually it wasn't all that early; they were still open because the weather was cool and damp.

This is the Bracted Spiderwort, Tradescantia bracteata. The flowers are about an inch across and are trimerous, with three green sepals, three petals (color varies), six stamens, and a capsule divided into three sections. (You're correct; this is monocot.)

Click on the image and look closely at the unopened sepals of the buds. The mix of glandular and eglandular (not gland-tipped) hairs is diagnostic of this species.

In his wonderful Jewels of the Plains, Claude Barr noted that flowers of Bracted Spiderwort range from "sky-blue to mauve-blue to very dark, from light lavender through orchid and heliotrope to deep purple, and from pale pink, near apple-blossom, to deeper tones bordering on purplish red. (3) Occasional albinos are found." And all this can be within one small area of prairie! The spiderworts I saw were lavender to pink, with one albino.

Note the long green leaves, one on each side; toe of botanist's shoe for scale.

White spiderwort flower, a nice bonus.

Geum triflorum, a plant of many names.

One of the more common wildflowers in the Castles grasslands is Geum triflorum—also known as Three-sisters, Long-plumed Avens, Old Man’s Whiskers, Lion's Beard, and most commonly here, Prairie Smoke. The multiple names in popular use may be due to its wide range. It grows across much of Canada, south through the western US into Mexico, and across the northern part of the US to the east. But its ubiquity doesn't make it any less wonderful to find, whether in flower or fruit.

In bloom, Geum triflorum is easily recognized by its nodding reddish flowers, three per stem. These are the Three Sisters, and the basis for the specific epithet, triflorum. The small petals are cream to yellow, and usually are mostly hidden inside the reddish sepals. On each sepal is a slender spreading to recurved bract.

Flower parts are nicely shown in this photo by Matt Lavin (Flickr; cropped).

Things change dramatically later in the season as the seeds mature. The flowering stems straighten and the styles elongate, becoming erect clusters of feathery-tailed achenes ready to fly away with the wind. These are the Old Man's Whiskers, or Lions' Beards if you prefer. This is when Geum triflorum makes its presence known. The many fuzzy heads visible above the grass look like smoke.

Click image to view plumose "seed tails"—persistent elongate styles. Photo by Matt Lavin (Flickr; cropped).

Prairie Smoke. Photo by Matt Lavin (Flickr).

This is a small sampling of the wildflowers I saw amid the Castles, and surely a tiny fraction of what is out there, especially through the seasons. I look forward to returning. And I must thank the few friends who knew of Slim Buttes for encouraging me to visit.

Notes

(1) Some readers familiar with this plant may be tisk-tisking. The accepted name now is Toxicoscordion venenosum var. gramineum. But because more than a few of us are still trying to remember the new genus name, I used the older one here. Note that the new name doubly emphasizes the plant's toxicity. 

(2) Maybe you're like I was, curious as to whether grasses—one of the largest families of monocots—have trimerous flowers. I checked. Sure enough, the much-evolved grass flower is still a bit trimerous, with three stamens and a three-parted ovary.

(3) Native plants master gardener Claude Barr had an eye for variety, and collected and cultivated novel forms that he found in the field. This may explain his lengthy list of flower colors for our spiderwort.

Sources

Barr, Claude A. 1983. Jewels of the Plains. U. Minnesota Press.

Clark, Frances. What’s in Bloom on Sageflats and Sunny Foothills – Late June 2022. Teton Chapter, Wyoming Native Plant Society.

Flora of North America online.

Missouri Botanical Garden. Plant Finder.

Ode, David J. 2006. Dakota Flora; a seasonal sampler. SD State Historical Society Press.

USDA Forest Service. Fire Effects Information System (FEIS, online).