Island of Grass

Island of Grass

by Ellen E. Wohl


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Product Details

ISBN-13: 9780870819636
Publisher: University Press of Colorado
Publication date: 10/15/2009
Pages: 296
Product dimensions: 5.76(w) x 8.74(h) x 0.83(d)

About the Author

Ellen Wohl teaches geology at Colorado State University and is the author of six other books.

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Island of Grass

By Ellen Wohl

University Press of Colorado

Copyright © 2009 University Press of Colorado
All rights reserved.
ISBN: 978-0-87081-963-6



[T]he ocean [in the central continent] is one of grass, and the shores are the crests of the mountain ranges, and the dark pine forests of sub-Arctic regions. The great ocean itself does not present more infinite variety than does this prairie-ocean. ... In winter, a dazzling surface of purest snow; in early summer, a vast expanse of grass and pale pink roses; in autumn too often a wild sea of raging fire.


Native grasses once sent up green shoots each spring from Alberta and Saskatchewan all the way south into Texas and the plains of Mexico. Grasses swayed in the prairie winds from the high plains of Montana east to the swampy lowlands of Illinois. Across the center of North America, 1.4 million square miles of grass supported immense herds of bison and bird migrations that darkened the skies. What Americans now sometimes call the breadbasket was a province of grasses: 46,000 square miles in the state of Iowa alone, and 40 percent of the continental United States, dominated by grasses. This was the landscape the first people of European descent to reach the center of the continent described as a sea of grass. One of the earliest written descriptions of the central Great Plains comes from Edwin James of the Long Expedition, who wrote while crossing the plains east of Council Bluffs, Iowa, in May 1820:

For a few days the weather had been fine, with cool breezes, and broken, flying clouds. The shadows of these, coursing rapidly over the plain, seemed to put the whole in motion, and we appeared to ourselves as if riding on the unquiet billows of the ocean. The surface is ... not inaptly called rolling, and will certainly bear a comparison to the waves of an agitated sea. The distant shores and promontories of woodland, with here and there an insular grove of trees, rendered the illusion more complete.

The metaphor of an inland sea of grass is so evocative that countless writers have used it since. Nineteenth-century writer Bayard Taylor described "broad swells of soil" with "long, wavelike crests." Isabella Bird wrote of the shortgrass prairie of eastern Colorado as "rolling in long undulations, like the waves of a sea which had fallen asleep." This metaphor works at many levels. From 100 to 80 million years ago the region now known as the Great Plains was the Cretaceous Interior Seaway, a shallow ocean with a geographic extent that coincides well with that of the historical sea of grass.

Because central North America is a landscape of grasses, it is also a spacious landscape of long views and broad skies. A person standing upright or seated on a horse has a much greater sense of the movement of clouds and winds across the grasslands than does someone in a forest. This landscape of distances impressed people of European descent differently. Those born elsewhere who visited the region as adults were as likely to be repelled as attracted. Crossing the shortgrass prairie of Wyoming and Colorado in September 1873, Englishwoman Isabella Bird saw the grassland as a landscape of absence: "The surrounding plains were endless and verdureless. The scanty grasses were long ago turned into suncured hay by the fierce summer heats. There is neither tree nor bush, the sky is grey, the earth buff, the air blae and windy, and clouds of coarse granitic dust sweep across the prairie."

Crossing the same region on a trip west from New York in 1859, Horace Greeley wrote, "[T]his is a region of sterility and thirst." Edwin James wrote of the western plains that "[t]he monotony of a vast unbroken plain ... is little less tiresome to the eye, and fatiguing to the spirit, than the dreary solitude of the ocean."

Those born to the grasslands, however, became its great poets. Remembering the Saskatchewan prairie, Wallace Stegner wrote: "The drama of this landscape is in the sky, pouring with light and always moving. The earth is passive. And yet the beauty I am struck by, both as present fact and as revived memory, is a fusion: this sky would not be so spectacular without this earth to change and glow and darken under it." Willa Cather remembered the Nebraska prairie of her childhood, where "the grass was the country, as the water is the sea. The red of the grass made all the great prairie the color of wine-stains, or of certain seaweeds when they are first washed up. And there was so much motion in it; the whole country seemed, somehow, to be running."

Cather's sense of motion in the landscape is apparent to anyone who has seen a broad sweep of closely growing grass stalks moving beneath the wind. But the sense of motion might just as well come from the evolutionary history of the grasses that cover the subdued topography of the Great Plains. Fossil records suggest that grasses originated in Africa. These adaptable plants have made good use of the 60 million years since their first appearance. Ten thousand species of grass have spread around the world, growing on every continent but Antarctica. The grasses do so well because they thrive in the vast interiors of the great landmasses of North America, Africa, and Eurasia, where it is too dry for trees.

Vascular plants such as grasses depend on stomata, tiny openings on each leaf through which the plant exchanges oxygen and carbon dioxide with the atmosphere. But the stomata are leaky; the great majority of water that leaves the plant escapes through the stomata. Water limits plant growth for at least some portion of the time in most environments. A plant's leaf must therefore compromise between exposing the maximum photosynthetic surface to the sunlight and conserving water. Plants have enormous leaves in tropical rain forests, where water is not such a limitation but the competition for sunlight is intense. In the abundant sunshine and extremely limited water of a desert, leaves can become unrecognizable as the spines of a cactus.

Grasses' narrow leaves minimize potential water loss with fewer stomata. The blades can also be ridged or covered in tiny hairs that create a rougher surface that holds a minuscule layer of moister air beside the leaf, helping to reduce the water lost when stomata are open. Dryland grasses further minimize water losses by being partially nocturnal. The grasses keep their stomata closed during the heat of the day and manufacture sugars and other molecules needed for growth during the cooler, moister evening, when the stomata open to take in carbon dioxide. Some species also roll up the edges of their leaves during dry times to reduce water lost through the stomata.

These physiological adaptations of grasses to dry conditions can be apparent to anyone who looks closely and carefully. More subtle but equally effective adaptations lie hidden within the biochemical reactions by which grasses live. Dryland grasses evolved a different metabolic pathway than grasses of wetter regions. In the original, so-called C metabolic pathway, the plant converts carbon dioxide to a sugar-phosphate compound via three "turns" of a metabolic cycle. A molecule of carbon dioxide enters the cycle at each turn and is converted into a new compound by enzymes within the plant. C plants growing in drier regions use a different metabolic pathway that requires more energy from the plant but also allows the plant to take up more carbon dioxide when the stomata are open. This shorter "breathing" time reduces the amount of time water can be lost from the plant. Drought-adapted C plants have a unique enzyme that only functions under relatively high temperatures in the range of 75° to 85°F. These so-called warm season grasses, such as blue grama, sand dropseed, and big and little bluestem, grow and produce seed in mid to late summer and are particularly common on the southern Great Plains. The C cool season grasses, such as needle-and-thread and western wheatgrass, that are more common on the northern Great Plains produce seed early in the summer before temperatures reach their annual maximum. Survival of all species of grasses improves in stands of mixed C and C plants that exert their maximum demand on scarce water supplies during different phases of the growing season.

The grasses of the Great Plains must conserve water so carefully because the combined effects of geologic history and atmospheric circulation allow very little precipitation to reach the grasslands during much of the year. What Edwin James described in 1819 as "that great Sandy Desert, which stretches eastward from the base of the Rocky Mountains" to the Platte River, results from two simple facts: the dominant movement of air across the planet is from west to east, and western North America is very mountainous. To fully understand these effects, we must look back into the unimaginably long spans of geologic history


Geologists commonly refer to the center of North America as the stable craton. The craton is the Earth's oldest geologic province and covers most of Canada and the central half of the continental United States. The region has been without mountain building, metamor-phism, or volcanism — all the surface signs of the Earth's restless interior — for more than a billion years. At the foundation of the craton is the basement composed of crystalline rocks that record a more exciting time in the geologic history of what is now central North America. The basement is a mosaic of rocks created in ancient volcanic island arcs and other fragments of the Earth's crust that collided with one another to form the nucleus of what is now North America. These collisions and coalescences occurred between approximately 3 billion and 1.5 billion years ago, reaching a crescendo during the Great Plains Orogeny of 1.8 to 1.6 billion years ago. Orogeny comes from the Greek words oros for mountain and geneia meaning "forming." Geologists coined the word to describe a period of mountain building. The Great Plains Orogeny occurred when the crustal fragments joining to form North America collided with such force that they uplifted mountains where the Great Plains are today.

That was the topographic high point of Great Plains history. The low point came soon after, when a north-south rift zone from Kansas to the Great Lakes stretched and thinned the proto–North America. Between 1.2 and 1 billion years ago, the crust grew so thin that basalts erupted along a mid-continent belt. The surface remained at low elevations for millions of years as the rift zone grew inactive. Oceans flooded in to occupy the low- lying center. Beginning about 500 million years ago, shallow seas repeatedly covered North America's interior.

Subsequent episodes of mountain building shifted farther west as North America grew from the agglomeration of crustal fragments slamming into the western boundary. The Earth's surface continually moves as tectonic plates shift in different directions. Crustal plate underlying an ocean basin tends to be thinner and denser than the plate underlying a continent. When an oceanic and a continental plate collide, the oceanic plate is typically forced back into the Earth's interior in a process known as subduction. Oceanic plate has been subducted along the western boundary of North America for hundreds of millions of years. As the oceanic plate descends and starts to melt in the subterranean heat, great masses of molten rock move back toward the surface to create volcanic archipelagos like Japan and the Philippines today. The steadily moving oceanic plate carries these archipelagos along until they collide with the continental plate and are accreted onto its margin. The Great Plains have thus gotten further and further from the nourishing moisture of the Pacific Ocean as North America has grown westward through geologic time.

The drama of mountain building continued along both the eastern and western margins of the central plains after the Great Plains Orogeny. The eastern margin of North America collided with Europe, Africa, and South America between 470 and 250 million years ago. The force of that collision created mountains the size of the Himalaya, which subsequently eroded to form today's Appalachians to the east of the Great Plains. Subterranean convective currents bringing heat upward from the Earth's interior shifted over time. North America separated from Europe and Africa and moved west, colliding with other plates en route. These collisions resulted in the formation west of the Great Plains of the mountainous Cordillera from Alaska south to Panama and beyond.

Wind, water, and ice can reduce the rock of mountain ranges into sediment and redistribute that sediment to adjacent lower elevations. Over millions of years, these forces spread a mantle of sediment derived from the Appalachians and the Rockies across what are now the central plains. Five hundred million years of mostly horizontal rock units record this long period of tectonic quiet in the continental interior. Sediment deposited in the interior ocean alternated through time with sediment deposited along rivers, on alluvial fans, and in coastal deltas and wetlands. Several thousand feet of layered sedimentary rocks now create a thick cover for the older basement rocks of the Great Plains.

The last of the seas receded from the North American interior by 65 million years ago, as the western boundary of the plains went through yet another upheaval. The Rocky Mountain Foreland Ranges — the eastern front of the great mountain mass known as the Rockies — rose between 75 and 45 million years ago. This episode, known as the Laramide Orogeny, resulted from the subduction of another tectonic plate beneath the edge of the North American Plate. The subducting plate pushed from the west-southwest, compressing the North American Plate and forcing its edges up into mountain ranges, just as pushing against the edge of a large carpet wrinkles that edge into folds and bulges. As the Foreland Ranges rose, water and wind carried sediment east onto the lowlands of the western plains. It can be difficult to realize this when traveling across the seemingly flat surface landscape of today's western Great Plains, but millions of years of weathering and erosion of the Foreland Ranges produced a massive wedge of sedimentary rock that thins from the source at the mountains toward the east. The thickness of sedimentary rock is reflected in ground elevations and topography; the plains are at 5,000 feet elevation at the base of the Rockies, where streams have cut canyons into the sedimentary rocks, but they drop to 1,000 feet elevation on the Missouri River. The landscape looks flat only because this 4,000-foot vertical drop occurs so gradually and over such long horizontal distances.

Uplift of the Rockies during the Laramide Orogeny created such a huge source of material that up to 20,000 feet of sediment was deposited in adjacent low-lying areas. Valleys between individual mountain ranges had filled sufficiently by approximately 38 million years ago for this sediment to be carried eastward across the Great Plains in three giant pulses of sedimentation that produced the rocks of the White River Group, the Arikaree Group, and the Ogallala Group. Each pulse of sediment overrode the deposits below and extended further east, creating layers of gravel, sand, and mud where braided rivers or migrating sand dunes left the sediment behind. Thin layers of ash among these sediments record the eruptions that rocked the lands further west as the heat source that today lies beneath Yellowstone National Park sent plumes of fine volcanic dust up into the atmosphere for the winds to carry and then drop to the east. Sometimes the ash falling from the sky killed and buried animals, as did the historical eruption at Pompeii. As a result, geologists know that rhinoceros, camels, ancestral elephants, diminutive three-toed horses, wolves, and saber-toothed cats roamed the ancient Great Plains.

The relentless movement of sediment from higher elevations toward lower elevations started to bury the Rockies, covering the lower parts of the ranges in a process that presumably would have continued until the region was nearly level if the rivers had not begun to cut downward, a process that continues today. Geologists debate exactly why the rivers began to incise 5 million years ago. One possibility is that renewed uplift of the mountains created steeper gradients and gave the rivers more erosive power. Another possibility is a change in climate to wetter and cooler conditions that produced more stream flow and more erosive power. Either way, the rivers saved the Rockies from oblivion beneath their own erosional products.

Climatic shifts occurred while the continents were colliding with one another like slow-motion bumper cars. The Great Plains have generally grown progressively cooler and drier during the past 65 million years. The retreating oceans left lands colonized first by tropical forests, which subsequently gave way to plants more tolerant of drought and cold; next by tropical evergreen trees, then seasonably deciduous trees, then shrubs, and finally broad-leaved herbaceous plants and grasses. Grasses became the dominant plants sometime after 15 million years ago, when fossil records indicate that the plains changed from partially open forests to completely open grassland. Species such as buffalo grass, which now extends from the southern tip of the Chihuahuan Desert north almost to the Canadian border, colonized the Great Plains from Mexico. Animals including the kit fox, jackrabbit, prairie dog, and coyote also apparently moved north from the Mexican plateau. Some of this evolutionary history is inadvertently reflected in the English word "coyote," which is derived from the Aztec word "coyotl."

Climatic cooling accelerated during periods of glacial advance. Continental ice sheets hundreds of feet thick covered the northeastern fifth of the Great Plains four times during the past 2 million years. The present-day course of the Missouri River approximates the southern boundary of the ice sheets from the foothills of the Rockies in northwestern Montana eastward into Nebraska and Kansas. Smaller glaciers moved down the valleys of the Rockies simultaneous with the advance of the continental ice sheets, but the Rockies were too dry to give rise to massive ice sheets like the one centered over Canada. Each summer, meltwater from the valley glaciers fed large rivers flowing eastward onto the plains.


Excerpted from Island of Grass by Ellen Wohl. Copyright © 2009 University Press of Colorado. Excerpted by permission of University Press of Colorado.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents


List of Figures,
1. The Sea of Grass,
2. Islands and Archipelagos,
3. Grassroots,
4. Life among the Leaves of Grass,
5. Keystone of the Shortgrass Prairie,
6. Hunters of the Grasslands,
7. On the Prairie Winds,
Glossary of Common and Scientific Names of Animals and Plants Described in the Text,
A Partial List of U.S. and Canadian Grassland Preserves,

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