River of Rock

Strolling down a river ...

Our fall was unprecedentedly mild—so much so that on December 9, I returned to the Pawnee Buttes in northeastern Colorado (see recent geo-challenge) to hike to the crest of Lipps Bluff. I wanted to walk where rhinos walked, stroll where camels strolled, and pause where oreodonts paused to drink.

Restoration of Merycochoerus, an oreodont, by Robert Bruce Horsfall, 1913. It's also called a ruminating hog, though not closely related to pigs.

The Pawnee Buttes and Lipps Bluff are relics of a time when the High Plains extended further west. In fact, about five million years ago the Plains reached all the way to the Rocky Mountains. Since then the South Platte River and its tributaries have cut down, eroded and carried off enough material to create the Colorado Piedmont, a lower area between the Rockies and today’s High Plains. [The Gangplank in southeast Wyoming is an exception.]

The Pawnee Buttes are in the northeast corner of the Colorado Piedmont, close to the High Plains. Map modified from Trimble 1980.

Escarpments mark the edges of the retreating High Plains. Sometimes buttes and ridges stand nearby—isolated remnants spared by erosion.

High Plains Escarpment northwest of Pawnee Buttes. This is wind country, shown by turbines above the escarpment and dark orthogonal lines below—tumbleweeds caught on barbed-wire fences.

Lipps Bluff and Pawnee Buttes stand above the Colorado Piedmont; High Plains Escarpment visible in the distance between the two buttes.

Harder rocks cap the Buttes and Bluff, slowing erosion. They're made of river deposits—gravel, sand and silt. In other words, the remains of a stream bed now form the high points of the landscape. This is a great example of topographic inversion.

Inverted topography is an awesome thing. To look down a 20-million-year-old stream bed, now 200 feet above the valley bottom, feels magical! It's experiences like this that make geotripping so exciting.

Sediments deposited by a stream roughly 20 million years ago now form the top of Lipps Bluff.

When I visited last spring, the Lipps Bluff trail was closed to protect nesting raptors (March 1 to June 30 every year). So I made another trip to check out the rocks on the crest. It was a short hike—maybe two miles roundtrip. But if you go, allow plenty of time for geo-gawking, plant appreciation, view-inspired contemplation and time-travel.

Lipps Bluff from the trailhead; tops of Pawnee Buttes visible behind on left.

On a map of old trails and settlements in northeast Colorado, West and East Pawnee Buttes are inexplicably labeled Devils Smoke House and Gabriels Castle (Scott 1989).

The rocks capping the Buttes and Bluff are part of the Ogallala Group, which also covers the High Plains (but often buried under recent deposits). It's the uppermost part of a giant wedge of sediments eroded off the Rocky Mountains and deposited to the east. The Ogallala Group represents the last of three major pulses of erosion and deposition, and the most extensive—reaching as far as eastern Nebraska and south across Texas. Deposition took place roughly 19 to 5 million years ago (Miocene).

Three major pulses of deposition of material eroded from the Rocky Mountains (Trimble 1980). The Ogallala occurs not just in the yellow area, but also on top of the older orange and brown units.

Being stream deposits, Ogallala sediments are heterogenous, ranging from fine silt deposited in slow waters to large cobbles or even boulders in raging torrents. Sequences are complex. Sediment size can change dramatically even in one place through a single year. And Ogallala rivers shifted around. Rivers move by cutting into banks, depositing sediments, and sometimes abandoning a channel entirely. Years later they may return, cutting into sediments they deposited earlier. So figuring out Ogallala rocks takes a lot of time and effort—like working on a puzzle with seemingly endless pieces, many of which have the same pattern but aren’t from the same part of the puzzle.

“The simple notion of sedimentary rocks as flat uniform strata, commonly called layer-cake stratigraphy, is completely inadequate to unravel the detailed history of these sediments.” —Maher & colleagues (2003) on Tertiary sediments of the Great Plains

Fortunately for geotrippers, the Ogallala Group has received a lot of attention in the Pawnee Buttes area. See papers cited below, as well as Recommended Reading at the end of the post for more on the larger context.

The gray rocks atop Lipps Bluff are part of the Martin Canyon Formation—the basal unit of the Ogallala Group here (Tedford 2004, Prothero & Dold 2008). They’re lithified stream sediments deposited in a valley that extended east into northwest Nebraska (Scott 1982). They rest unconformably on siltstones of the Oligocene White River Group. Apparently the intervening Arikaree Group is missing.

Unconformity between the Martin Canyon Formation and pale siltstones of the White River Group.

Martin Canyon outcrops look very much like stream deposits. Here’s a sandy bar, and layers of cobbles deposited during faster flow.

Stream-deposited cross-bedded sand(stone).

A distinctive feature of the Martin Canyon Formation is beds of calcareous siltstone nodules, which look like little gray potatoes. The silt is from older strata, cemented with calcium carbonate.

Siltstone nodules in a matrix of finer sediments.

Broken nodule reveals its concentric nature.

Nodules weathered out, ready to travel again.

Martin Canyon clasts are a mix of locally-derived sedimentary rocks like the siltstone nodules, and crystalline rocks from the Rocky Mountains. I found pieces of pink Sherman granite from the Laramie Range, my home territory.

Then there were the many puzzling patterns in the rock—do they say something about the river, the environment, subsequent processing? Do you know?

Occasionally I found patches of blackened coarse sandstone, also a mystery.

From a geological perspective, Lipps Bluff and the Pawnee Buttes are not long for this world. Their river-rock caps are falling apart, so they too will disappear into the Colorado Piedmont.

Erosion of the soft White River slope undercuts the Martin Canyon cap, and fragments fall.

A souvenir—a fallen chunk of coarse sandstone with siltstone nodules (12" tiles).

Why would the Ogallala Group be thoroughly studied in the Pawnee Buttes area? No, not for oil and gas (good guess though). It’s because fossils are abundant. And there are great fossil sites scattered across the High Plains, which means we know a fair amount about life during Ogallala times. So let’s time-travel to the “Martin Canyon River” back when it was water rather than rock.

The setting wasn’t all that different from the High Plains today. Just fill in the zillion tons of dirt excavated by the South Platte, remove the wind turbines, rewind any cultivation, and maybe tweak the composition of the sweeping grasslands that went on forever. It seems there was a more pronounced dry season during the Miocene (Maher & colleagues 2003).

Yuccas were common in Ogallala times, as they are today.

In the distance we would see herds of animals grazing—like the herds of bison of the recent past and cattle today. But if we hid among the hackberries along the river at dusk, we’d see that these animals are strangers. Rhinos, small camels, tiny horses, strange horse-like creatures with claws, and pig-like oreodonts would come to drink in the dim light, glancing up constantly, looking this way and that, watching for movement, ready to run from the dreaded bear dogs.

Small rhinos (Menoceras ) were common on Miocene grasslands. Restoration by Robert Bruce Horsfall, 1913.

Parahippus, an extinct relative of today’s horse, stood about a meter tall. Unlike its ancestors, it was a grazer and well-adapted to life on the plains. Restoration from a Smithsonian mural, 1964.

Another common inhabitant was Moropus, perhaps related to the modern horse, rhino, and tapir. In other words, it was odd. It had long claws for defense or maybe for digging. Restoration of Moropus threatening a pair of bear dogs, by Jay Matternes.

Daphoenodon was a large bear dog (not a canid), a long-legged pursuit predator adapted to the open plains. Restoration by Robert Bruce Horsfall, 1913.

Recommended Reading

The High Plains: Full of Character provides an excellent summary of the bigger picture—the High Plains in the context of the Great Plains.

Roadside Geology of Nebraska (Mayer & colleagues 2003) gives lie to the belief that Great Plains geology is boring. And it’s not your typical roadside geology guide. It starts with a thorough introduction, and includes detailed descriptions of areas of interest.

Erosion in a Land of Sky and Grass

Last week I was faced with a trip to Fort Collins.  Laramie isn’t quite big enough to satisfy every material desire, so occasionally I brave the urban sprawl and crazy traffic below the Colorado Front Range to do some shopping.  As an incentive I planned a geo-trip afterwards.  The theme was erosion.

Front Range of the Rocky Mountains above Fort Collins, Colorado.  Source.

The spectacular Front Range always impresses me, even with the explosion of tech centers and bedroom communities below.  I contemplated its geological history as I drove south on I-25.  Think mountains, think uplift … but erosion usually is just as important.  In this case, two major episodes of erosion contributed to the landscape, and not just in the mountains but also far to the east.

The Front Range is part of the eastern flank of the Rocky Mountains.  Uplift began roughly 60  million years ago.  Erosion set in almost immediately and continued off and on for roughly 50 million years.  First the cover of sedimentary rocks was removed, then underlying hard rocks were sculpted into today’s charismatic alpen forms.  Massive amounts of debris were carried off in the process, deposited as far away as eastern Nebraska and Kansas.  A huge debris apron was created – a giant wedge of sediments sloping to the east.  At its maximum, the apron was so thick that the mountains were largely buried, in their own detritus.  Only the high peaks were exposed.  The Rockies were just scattered minor mountain ranges.

Block diagram showing a mountain range mostly buried in Tertiary sediments (yellow); just the high peaks remain.  B Mears Jr, after SH Knight; Geomorphology class, University of Wyoming, 1984.

Then things changed, as things do, and a new episode of erosion restored the glory of the Front Range.  Streams cut into and carried away the sediments of the debris apron.  The mountains were exhumed, probably in less than two million years.  What changed?  The cause of the shift to erosion is debated.  Perhaps the climate became wetter, perhaps the entire region was uplifted, perhaps both.

Exhumation didn’t stop at the base of the mountains.  Currently the debris apron has been removed almost to Nebraska, Kansas and Wyoming.  This excavated area is the Colorado Piedmont.  Beyond, the debris apron is still in place and forms the High Plains.

Physiographic provinces of Colorado; Colorado Piedmont and High Plains are subunits of the Great Plains Province.  After Matthews 2009.

From Fort Collins I drove east across the Colorado Piedmont through bedroom communities and heavy traffic to Greeley for information (more on this later), and then northeast toward the High Plains Escarpment.  The transition from Piedmont to High Plains is often scenic, where erosion has carved arroyos, escarpments, cliffs, breaks and isolated buttes.

My destinations were Pawnee Buttes and the surrounding Pawnee National Grassland, public land where I could freely wander.  It was added to our network of public lands in 1960, along with 19 other National Grasslands, but its origins date to the late 1880s when homesteaders began to settle the western High Plains.  For 50 years, people full of hope – but badly misled – tried to survive in this harsh country.  They met with disaster after disaster.  Cattle died in blizzards, wheat burned in prairie fires, tornados destroyed homes.  Worst of all, the rain didn’t come the way the railroad companies, eager for customers, had told them it would.

Folks settled here on the promise of dryland farming, with all water supplied by rainfall.  But the rain was unreliable.  It varied, greatly.  The first bad drought lasted from 1880 to 1890; there was another in ’93 and ’94.  Then dreams came true.  Rainfall was sufficient, even abundant.  Farmers raised bumper crops, livestock thrived, towns grew.  But starting in 1931 there was so little rain that plants in the plowed fields and heavily-grazed pastures simply shriveled up and died.  Then the soil blew away.

These were the Dirty Thirties, the Dust Bowl years.  People went bust.  Towns shrank.  Some disappeared entirely.  The Federal government stepped in and bought out the destitute farmers, paying them for their land.  With rest and revegetation, the worn-out land recovered.  It became Pawnee National Grassland in 1960.

I knew my plan was a good one when I stopped at the Grassland’s District Office in Greeley.  It was small and a bit worn.  Inside, a lone employee was able to answer all my questions.  He offered free brochures on history, wildflowers, birds, paleontology and more, “available outside when we’re closed.”

My enthusiasm grew as I drove east.  Traffic dropped off dramatically and I could ponder the landscape.  Bedroom communities were replaced with rolling terrain dotted by occasional oil and gas facilities (part of the oil-rich Denver Basin lies below the surface).  The stress of Fort Collins faded from memory.

It was a land of sky and grass, both so expansive that photographs were unthinkable.  Beauty and awesomeness came from the immense vistas, which seem to go on forever … maybe even to the edge of the world!  It was impossible to capture them in images; you will just have to believe me.

The light and air about me told me that the world ended here: only the ground and sun and sky were left, and if one went a little farther there would only be sun and sky, and one would float off into them, like the tawny hawks which sailed over our heads making slow shadows on the grass.  Willa Cather in My Antonia

Pawnee National Grassland is a patchwork of public and private land traversed by gravel roads.  A Travel Map (free at the District Office) is essential if you want to wander without trespassing.  However the route to the Buttes was clearly signed, no map needed.  It led to a small parking lot, picnic shelter, vault toilets and an interesting and scenic hike.

The Pawnee Buttes and small ridges nearby are relics of the High Plains Escarpment, which is now a short distance to the north and east.  Rocks exposed here date from the Tertiary Period, as do all strata of the debris apron (diagram below).  Most are relatively-unconsolidated, soft, and easily-eroded.  The Buttes and ridges still stand because resistant rock caps protect them from erosion … for now.

Three groups of Tertiary sedimentary rocks represent three major pulses of deposition of mountain debris.  All are exposed in the Pawnee Buttes area.  After Maher et al. 2003.

The out-and-back trail to the Buttes crosses grassland, descends an eroding drainage, winds around to the north side of Lipps Bluff, and continues east to the Buttes (three miles round trip).  A short distance from the parking lot the trail splits, with one fork crossing Lipps Bluff.  When I visited, it was closed to protect nesting raptors.

The Buttes are 200 to 300 feet tall.  Erosion-resistant caps of conglomerate and sandstone (Oglalla Group) protect underlying mudstones and siltstones.

Erosion was evident everywhere, especially along the trail.

The trail is reinforced where terrain is steep.  Log steps are common.

I liked the patterns of short deep gullies in soft rock.  This is badlands topography – rapidly changing, sparsely vegetated.

The landscapes here are at a scale suitable for photos … thanks to erosion.

Note protective cap on Lipp's Bluff.  I look forward to viewing it up close later this year.

West Butte’s shadow crosses the High Plains Escarpment; wind farm beyond.

The growing season had barely started; grasses were mostly brown and only a few early wildflowers were blooming.  Peak season for spring wildflowers starts in another month or so, though with the drought this year it may be underwhelming.  A report on plants of the Grassland is available here (PDF).

Stands of Great Plains yucca indicate sandy soils.

My favorite spring flower, the Easter daisy (Townsendia).  It’s a small plant but the flower heads are relatively large; when everything else is brown, it seems downright showy.

It was a really fun hike!

I was pleased with my trip to Pawnee National Grassland, a welcome respite from the frenzy to the west.  The hike to the Buttes was scenic and peaceful, with lots to see and think about.  I plan to return when spring wildflowers are in full bloom, and again when the Lipps Bluff trail is open.  

Given my enthusiasm for the area, I may be misleading the reader.  The Grassland is hardly wilderness; in fact it’s busy on weekends.  Until recently most human activity centered on natural resources – grazing, wind farms, oil and gas extraction.  Now recreational use has exploded with population growth along the Front Range, leading to intensive management. But there are benefits … a birding route, natural history brochures, a campground and trails just for hiking.  A new target-shooting range has reduced the dangers of dispersed shooting. Off-road vehicles are restricted to designated routes.  Still, it’s probably best to visit during the week.

Sources

Maher, HD, Jr., Egelmann, GF, and Shuster, RD.  2003.  Roadside geology of Nebraska.  Mountain Press Publishing Company.  NOTE: This is an exceptional road guide, with detailed explanations of Great Plains geology.

Matthews, V.  2009.  Messages in stone: Colorado’s colorful geology, 2nd ed.  Colorado Geological Survey.

Rhodes, Dorothy and Lee.  No date (after 1986).  A history of the Pawnee National Grassland (brochure).  Available from USDA Forest Service, Pawnee National Grassland.

USDA Forest Service.  No date.  An introduction to the Pawnee Buttes (brochure).  Available from USDA Forest Service, Pawnee National Grassland.