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The Prehistoric Prairie

My eyes followed the contour of the land, the rolling hills, swales, creeks, arroyos, and the expanse of prairie grass and cactus. The wind stirred, creating invisible waves swirling through the grasses, while the clouds slowly drifted eastward, growing into afternoon thunderheads. I knew that migrating birds would soon reappear, some stopping only for a brief time before continuing on a longer journey. Despite such natural changes, the Chico Basin has a sense of timelessness about it, a permanence as though is has always been here and always will be. 

Yet the prairie was not always as it looks today. The land has changed dramatically over the long arc of time. Changes occur continuously, albeit slowly, in minute, imperceptible increments, sometimes brought about by a sequence of serendipitous shifts in geology and climate. Yet, sometimes, changes are rare, rapid, and cataclysmic on a grand geologic scale. Life has been disrupted, even obliterated, numerous times, only to rebound, reemerge, and adapt to the new norm. Evidence of these great events along with the remains of past life are recorded, etched in stone, and compressed in rock layers like pages in the book of time. However, often these records are erased as layers of rock are exposed, eroded, and swept away.

On the far western side of the ranch stand several cone-shaped hills overlooking the surrounding prairie. These are ancient sea vents that formed on the bottom of a seabed. The nutrient-rich effluvia from the vents promoted a wealth of marine life creating a buildup of fossil-rich limestone. Climbing to the top of one of these cones, one can find an abundance of corals, clamshells and other bivalves, and numerous other fossils. Last August, after leaving the sea vents, I drove south past the old Bar JH house and forded the creek in my Jeep to the east side and then turned south again. I parked the Jeep and hiked down to the creek bottom. From there I walked downstream along the gravel and sand bar that one long-time Chico Basin resident calls the Beach. I kept my eyes on the ground, looking for fossils or anything else of interest. I found numerous pieces of fossilized coral, tubeworms, and clamshells strewn across the sand, several fossil remnants of belemnites and baculites, ancient species of cephalopods, squid-like creatures. I reached down and picked up a rock that caught my eye. I turned it over in my hand. It appeared to be an impression of an ammonite, another type of marine cephalopod, with a shell much like a chambered nautilus.

The extent of the Western Interior Seaway 95 million years ago, overlaid on the modern North American continent. Both island continents Laramidia and Appalachia are shown. Note that the positions of the continents, lakes, rivers, and shores were not the same as today. Illustration by Andrew Z Colvin.

During the Cretaceous Period, this land was submerged, covered by the great Western Interior Seaway from roughly 100 million years ago (Mya) to 68 Mya. The sediments deposited in the sea formed the Pierre Shale formation thousands of feet thick and exposed in a few places along Chico Creek. Besides the multitude of mollusks, corals, and fish, large marine reptiles like mosasaurs, some reaching over 30 feet long, swam the Interior Seaway.

1872 Paleo-art depicting prehistoric sea animals discovered in the Great Plains by Henry Worrall.

Stepping further back in time, one billion years ago, molten magma from the Earth’s mantle intruded upward into the Earth’s crust to the northwest of Chico Basin Ranch. Remaining deep underground, it cooled slowly creating a granitic batholith. Eventually the huge batholith was uplifted during the Laramide Orogeny (a period of mountain building) starting in the Late Cretaceous from approximately 80 – 55 Mya. During this period, tectonic forces began to uplift the land in a series of pulses to form the Rocky Mountains. The uplift drained away the inland sea. Glaciers ground down the rock exposing the batholith. This is now known as Pikes Peak and the surrounding mountains that form our skyline today.

I was puzzled at first to find gravel and chunks of granite strewn across the surface of the prairie. These are remnants of the ancient rock, the Pikes Peak granite, lying above much younger layers of rock. These must be recent Quaternary gravels that had been ground up and carried thirty miles or more from the mountains by great alluvia spreading across the plains by the forces of ice and floods. Dunes cover other parts of the Chico, the result of sand carried by the wind over millennia.

After the sea had drain away about 68 Mya, the exposed land consisted of low-lying lakes, swamps, and marshes. Terrestrial life was limited to these areas, as plant life was dominated by a variety of spore-bearing ferns and similar trees in a much warmer climate. These plants require marshy wet areas for reproduction. Obviously, plant-eating dinosaurs, reptiles, amphibians, early mammals, and insects lived where the plants lived. The carnivores lived where the herbivores lived.  

It wasn’t until the development of seed-bearing plants that life could spread far and wide into drier and cooler regions. Once that happened, life spread across the land in great forests of conifers and deciduous trees. Although early seed-bearing grasses had evolved in the Cretaceous Period, it wasn’t until the uplift of the Rocky Mountains to the west that a major shift in climate resulted fostering the emergence of a new biome – grasslands. Surprisingly, this didn’t occur until around 5 Mya during the Late Miocene. These grasses sequestered high amounts of carbon and water in their root systems resulting in a further transition to a cooler and drier climate.

The skeleton of a sabre-tooth tiger. Photo via the American Museum of Natural History.

Grasslands transformed the landscape across the world and with it whole new families of animals evolved. As time passed, vast areas of grass developed an ability to extract silica from the soil, incorporating it into the fiber of the grass stems and leaves. Many animals could not chew and digest such grasses and went extinct. They were replaced by new species of grazers and ruminants that had high crowned teeth that could chew and grind the siliceous grasses – wooly mammoths, rhinoceroses, camels, horses, deer, bison, and pronghorn. Along with these emerged predators such as American cheetahs, American cave lions, genus Smilodon or sabretooth cats, dire wolves, and short-faced bears.

Paleolithic humans hunted these animals as far back as 20,000 or more years ago. Most megafauna species went extinct from North America at the end of the last ice age, about 10,000 years ago. In some cases, as with the short-faced bear and dire wolf, they were replaced by smaller, more agile species such as the grizzly bear and gray wolf, respectively. The hoofed animals that remained included the bison, wapiti (elk), moose, deer, bighorn sheep, and pronghorn.

Petrified bone likely from the Spinous Process of a Bison’s Vertebrae comprising part of the the hump. Bone is embedded with a piece of shale. Photo by Shane Morrison.

Last August, I was exploring along Chico Creek, when I came upon a small bone fragment. I dropped my pack and shovel and looked around carefully. I soon found several fossilized bone fragments. A couple of the fragments look like parts of a rib bone. The most prominent was a petrified bone about five inches long embedded in a chunk of hard shale. It resembled a rib but was straight. After researching several reference skeletons, I determined that the petrified bone is most likely the spinous process of a vertebra that formed the hump of a bison. Although I can’t rule out that the bone came the hump of a camelops, a North American camel, it more likely came from a modern bison, Bison bison, which has only been around for 5,000 years, or perhaps its predecessor Bison antiquus. Other bison artifacts including a skull have been found on the ranch. We think the cliff close to where I found this fossil bone may have been a buffalo jump, as other bison bones have been found there. A buffalo jump is a cliff or steep arroyo where Native Americans from 10,000 years ago to the early 1800s would stampede the bison herds, directing them over the edge of a cliff. Having survived the great Holocene extinction, bison once roamed across the Great Plains in the millions, yet the wild bison herds were extirpated from the plains by the late 1800s by hide hunters and the U.S. Army’s campaign to crush the Plains Indian cultures by eliminating their food supply.

The “beach” and eroded cliff face along Chico Creek. Photo by Shane Morrison.

The Great Plains wolf, or buffalo wolf, and the grizzly bear once roamed widely across the Great Plains, including the Chico Basin region. Based on accounts from native plains tribes, it only took three buffalo wolves to take down a bison. According to explorer John C. Fremont’s journals from his 1843 expedition, on July 11, 1843, his men put six rifle balls into a large Grizzly Bear before it dropped along Bijou Creek about 40 miles due north of the ranch. Hunters and settlers had exterminated the grizzly and wolf from the prairies by the late 1800s, although this was partially caused by the disappearance of the bison.

The elimination of the bison had a dramatic impact on the prairie ecosystem, eliminating the effects of intense grazing, aeration, and fertilization on the grasses. There used to be thousands of wallows, depressions made when the bison would roll in dust and mud to protect themselves from insects. These wallows would collect pools of rainwater providing habitat for amphibians, birds, and mammals. Numerous animals were dependent on the bison grasslands.

These great grasslands continue to dwindle today, steadily losing its biodiversity. Since the demise of the bison, much of the native short-grass prairie has been lost to agriculture, mining, urban sprawl, and freeways. A changing climate may further alter the face of the prairie as we know it. The land is changing, even if that change seems imperceptible.

A curious pronghorn family at Chico Basin Ranch. Photo by Shane Morrison.

I pondered these things as I stood and gazed, as I often do, across the Chico Basin and the expanse of grass. Here the short-grass prairie still thrives. I again felt a sense of timelessness. And while the bison, wolf, and grizzly no longer roam here, I was reassured, if only for a moment, to see a group of pronghorn running across the prairie, and in the knowledge that migrating birds will return next spring as they have always done since time immemorial.

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