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.

Winter on the Laramie River—great food, wrong trees

Would you camp here for five months?

On September 4, 1831, twenty-one fur trappers packed their mules, saddled their horses, and rode up the Laramie River from the North Platte. They would travel until they found beaver, then trap until snow and cold sent them back downstream. But things did not go as planned. It would be May before they finally returned, and they would have to walk back.

Among the trappers was 22-year-old Zenas Leonard, one of the many young men who joined the fur trade for economic gain and adventure. He left the family farm in Pennsylvania the year before after announcing, “I can make my living without picking stones.” Zenas was a literate man, and kept notes during his travels. In 1839, he published a book about his five-year journey through the Rocky Mountains: Narrative of the Adventures of Zenas Leonard (available free here). The following account of a winter on the Laramie River is based on Leonard’s Narrative.

NOT Zenas Leonard; added in error by GoogleBooks. See note at end of post.

Traveling was easy at first. “… found the prairies or plains very extensive—unobstructed with timber or brush—handsomely situated, with here and there a small creek passing through them, and in some places literally covered with game, such as Buffaloe, White and Black tailed deer, Grizzley, Red and White Bear, Elk, Prairie Dog, wild Goat, Big horned mountain Sheep, Antelope, &c.”

But when they arrived at “the foot of a great mountains through which the Laramies passes” they found it impossible to continue, as “huge rocks projecting several hundred feet high closed it to the very current.” Instead, they traveled along the base of the Laramie Range to a buffalo trail leading to the crest, where they made camp. At midnight it began snowing hard, and they were forced to stay put for three days.

Not bothered by the early-October blizzard, the party continued on to the Laramie Valley. Leonard described it as long and broad “with the river Laramies passing through the centre of it, the banks of which are covered with timber, from 1/4 to 1/2 a mile wide … on a clear morning, by taking a view with a spyglass, you can see the different kinds of game that inhabit these plains, such as Buffaloe, Bear, Deer, Elk, Antelope, Bighorn, Wolves, &c.”

Beaver were abundant; they trapped twenty the first night. Then they continued upstream, periodically stopping for a few days to trap. Clearly the Laramie Valley was worth the trouble of getting there.

Base map by John C. Fremont (drawn by Charles Preuss), 1848; from David Rumsey Historical Map Collection. Insert below; click on images for larger views.

Trappers' route from the confluence of the Laramie and North Platte Rivers to the Laramie River campsite (X). Precise locations of their route across the Laramie Mountains and campsite are unknown. At that time, the Laramie Mountains were often called the Black Hills.

By October 22, the days were consistently cold and snowy. All agreed it was time to return to winter quarters on the North Platte. They followed the Laramie River back downstream to the buffalo trail across the mountains. But … surprise! It was no longer passable—there was too much snow. Several men searched for an alternative route but found none. In the discussion that followed, “a majority of the company decided in favor of encamping in the valley for the winter.”

The river was the obvious place to stay. Game was abundant. Cottonwood trees would provide wood for shelters, fuel for heat, and nutritious inner bark for horses and mules when grass was buried in snow. Camp was established on November 4.

“… we arrived at a large grove of Cottonwood timber, which we deemed suitable for encamping in. Several weeks were spent in building houses, stables, &c. necessary for ourselves, and horses during the winter season. They [the best hunters] killed buffalo and dried meat in case the herds left the valley. They killed deer, elk, antelope and other game & dressed the hides to make moccasins.”

By early December, the horses were struggling to find grass. The men collected armloads of cottonwood bark, but “to our utter surprise and discomfiture, on presenting it to them they would not eat it, and upon examining it by tasting, we found it to be the bitter, instead of the sweet Cottonwood.” By the end of December, most of the horses had died (apparently the two mules were less picky). They celebrated the New Year anyway.

“On new-years day, notwithstanding our horses were nearly all dead, as being fully satisfied that the few that were yet living must die soon, we concluded to have a feast in our best style; for which purpose we made preparation by sending out four of our best hunters, to get a choice piece of meat for the occasion. These men killed ten Buffaloe, from which they selected one of the fattest humps they could find and brought in, and after roasting it handsomely before the fire, we all seated ourselves upon the ground, encircling, what we there called a splendid repast to dine upon. Feasting sumptuously, cracking a few jokes, taking a few rounds with our rifles, and wishing heartily for some liquor, having none at that place we spent the day.”

Food and fuel remained abundant, but the men grew restless. Someone had heard they could buy horses in Santa Fe, so all but four men headed south on foot with beaver skins to trade. Two weeks later, they were turned back by snow and dwindling supplies of food. By the time they returned to the Laramie River camp they were gaunt and hungry, but quickly fattened up on game.

Finally, on April 20, they loaded what they could on the two weakened mules, cached everything else, and headed east across the Laramie Mountains through deep snow. Back on the plains, they stopped at the first sweet cottonwoods they came to and let the mules feast on inner bark for several days. They reached the North Platte on May 20, 1832.

Why no one in the group recognized the Laramie River cottonwoods as the bitter type is puzzling. Travelers as far back as Lewis and Clark could distinguish between the sweet and bitter types, even without leaves, and knew that horses would not eat the bark of the latter.

Were they an ignorant bunch? After all, they crossed the snowy Laramie Range in October, trapped beaver in the Laramie Valley into early November, and rang in the New Year with gusto in spite of losing all their horses, intending to walk to Santa Fe to get more.

Or were these trappers skilled adventurous men not averse to hardship? Maybe for them it was no big deal to spend five wintry months camped on the Laramie River before walking back to the North Platte!

Bitter cottonwoods (left; now called narrowleaf cottonwoods) grow at higher elevations, including the Laramie Valley (7000 feet). Sweet cottonwoods (Plains cottonwoods) are trees of lower elevations.

ADDED NOTE (May 21, 2016): In digitizing Leonard's narrative, GoogleBooks added a portrait to the cover as an enhancement—but it was the wrong one! Thanks to Scott Stine for tracking down the source of the error, and notifying Google:

[the] "digitized version of the 1904 edition displays a portrait, labeled 'Zenas Leonard,' on the first (digitized) page. But that portrait is not of Zenas Leonard; rather, it is of Michel Sylvestre Cerré. The portrait comes from (opposite) page 146 of the 1904 book (which appears on page 143 of your digitized version), where it is correctly labeled 'Michel Sylvestre Cerré.'"

• • •

This is an expanded version of an article I wrote for Laramie's Living History—a series produced by the Laramie Boomerang and the Albany County Museum Coalition.

Hollis Marriott
Contributing History Columnist

Geo-challenge: hiking on (not in) a river valley

Where on Google Earth are we?

Some hints ...

Arrows point to stream deposits from roughly 20 million years ago. Obsolete names date from the late 1800s.

Stream deposits cap a ridge (inverted topography); Devils Smoke House and Gabriels Castle behind. Note spectacular unconformity.

The trail on the ridge crest passes through stream deposits turned to rock and photogenically eroded. 

Layers of sand, pebbles, cobbles record changes from about 20 million years ago.

Cemented layer, above siltstone nodules in fine matrix, above unconformity; cross-bedding near top of photo. 

Mostly siltstone nodules.

Crystalline clasts from the Rocky Mountains.

I found many puzzling weathered forms in cemented sand. Here are a few:

Happy dog for scale.

Recognize this place? Please contribute via a Comment. If the answer isn't in the Comments below, check back in a week or so.

Willow Retrospective

Limestone "pavement" above Willow Canyon, with windblown snow.

It’s a cold windy December afternoon in the foothills of the Laramie Mountains—7200 feet elevation, heart of the North American continent, far from any ameliorating marine effects. But no matter. A party of intrepid tree-followers sets off, headed for a small limestone canyon where a willow grows.

We’re not alone. Around the globe, tree-followers are checking their trees. This phenomenon began several years ago, launched by a visionary from coastal Dorset (UK), and now kindly hosted by The Squirrel Basket in Wales. It’s a fascinating project; we always find something of interest. And on the seventh of the month, everyone gathers to report the latest news of our trees (the list of links will grow through the week).

Approaching the canyon mouth.

The pussy willow I'm following grows in the canyon bottom, in a nook in the limestone wall, shaded by junipers. There's no way to capture the whole tree in one photo.

It was a good year for the willow, just look at the young growth—long reddish twigs at the ends of branchlets (below). Lots of flower buds too, ready to open in February, just a few months away! The North American pussy willow is one of the earliest plants to bloom here, maybe even the first. It beats most plants by several months or more.

But the willow won’t leaf out until late April or early May. Meanwhile the neighbor trees—evergreen junipers and limber pines—can catch a little solar energy on warmer days.

It looks like a few of the lip ferns cliff brakes (Pellaea) are still green. But barely, and I think this is just remnant green pigment. [Thanks to Plant Postings for pointing out the error in common name!]

Sagebrush stays green all winter—gray-green actually. That's its summer color too.

Here's a view of the willow's canopy from the canyon rim (dog tail for scale).

The willow sticks up above the rim just right of the juniper at the left edge of the photo.

I highly recommend getting to know a tree—consider joining us next year!

Thanks again to Lucy and Pat.