Part 2

Quaternary Pleistocene Glacial deposits, Alluvium, sand Piedmont, terrace, alluvium, and dunes, and loess and bolson terrace deposits deposits 2 erosional surface Tertiary Pliocene EROSION 5 Flaxville Gravel Ogallala formation and Ogallala Formation Miocene Arikaree Formation Arikaree Formation 22-24 erosional surface Oligocene White River Group White River Group Mostly missing because of erosion or nondeposition 37-38 erosional surface Eocene Wasatch and Golden Valley Formations 53-54 Dawson Arkose Paleocene Fort Union Denver, Poison Formation Canyon, and Raton Formations 65 Cretaceous Hell Creek and Vermejo and Laramie Lance Formations Formations Fox Hills Sandstone Trinidad and Fox Hills Sandstones Shales, sandstones, and limestones deposited in Late Cretaceous sea Dakota Sandstone Dakota Sandstone and Lakota Formation Glen Rose and Edwards Limestones 136 Jurassic Sundance Morrison Formation Jurassic rocks not Formation, Ellis present Group, and Unkpapa Sandstone 190-195 Triassic Dominantly red rocks 225 PALEOZOIC Paleozoic rocks, undivided 570 PRECAMBRIAN Precambrian rocks, undivided

As the mountains continued to rise, the eroding streams cut into the old core rocks of the mountains, and that debris too was carried to the flanks and onto the adjoining plains. The mountainous belt continued to rise intermittently, and volcanoes began to appear about 50 million years ago. Together, the mountains and volcanoes provided huge quantities of sediment, which the streams transported to the plains and deposited. The areas nearest the mountains were covered by sediments of Late Cretaceous and Paleocene age (table 1)—the Poison Canyon Formation to the south, the Dawson and Denver Formations in the Denver area, and the Fort Union Formation to the north (fig. 8). Vegetation continued to flourish, especially in the northern part of the Great Plains, and was buried to form the thick lignite and subbituminous coal beds of the Fort Union Formation (fig. 9). The earliest mammals, most of whose remains come from the Paleocene Fort Union Formation, have few modern survivors.

Beginning about 45 million years ago, in Eocene time, there was a long period of stability lasting perhaps 10 million years, when there was little uplift of the mountains and, therefore, little deposition on the plains. A widespread and strongly developed soil formed over much of the Great Plains during this period of stability. With renewed uplift and volcanism in the mountains at the end of this period, great quantities of sediment again were carried to the plains by streams and spread over the northern Great Plains and southeastward to the arch or divide separating the Williston and Anadarko basins (fig. 8). Those sediments form the White River Group, in which the South Dakota Badlands are carved. In addition to the _Titanotheres_, huge beasts with large, long horns on their snouts who lived only during the Oligocene (37 to 22 million years ago), vast herds of camels, rhinoceroses, horses, and tapirs—animals now found native only on other continents—grazed those Oligocene semiarid grassland plains.

Powder River basin Denver basin Raton basin PLAINS Margin of Oligocene deposition Margin of Miocene-Pliocene deposition

Sometime between 20 and 30 million years ago the streams began depositing sand and gravel beyond the divide, and, for another 10 million years or more, stream sediments of the Arikaree and Ogallala Formations spread over the entire Great Plains from Canada to Texas, except where mountainous areas such as the Black Hills stood above the plains. Between 5 and 10 million years ago, then, the entire Great Plains was an eastward-sloping depositional plain surmounted only by a few mountain masses. Horses, camels, rhinoceroses, and a strange horselike creature with clawed feet (called _Moropus_) lived on this plain.

SCULPTURING THE LAND

Sometime between 5 and 10 million years ago, however, a great change took place, apparently as a result of regional uplift of the entire western part of the continent. While before, the streams had been depositing sediment on the plains for more than 60 million years, building up a huge thickness of sedimentary rock layers, now the streams were forced to cut down into and excavate the sediments they had formerly deposited. As uplift continued—and it may still be continuing—the streams cut deeper and deeper into the layered stack and developed tributary systems that excavated broad areas. High divides were left between streams in some places, and broad plateaus were formed and remain in other places. The great central area was essentially untouched by erosion and remained standing above the dissected areas surrounding it as the escarpment-rimmed plateau that is the High Plains.

This downcutting and excavation by streams, then, which began between 5 and 10 million years ago, roughed out the landscape of the Great Plains and created the sections we call the Missouri Plateau, the Colorado Piedmont, the Pecos Valley, the Edwards Plateau, and the Plains Border Section. Nearly all the individual landforms that now attract the eye have been created by geologic processes during the last 2 million years. It truly is a young landscape.

LANDFORMS OF TODAY—The surface features of the Great Plains

The mountainous sections of the Great Plains were formed long before the remaining areas were outlined by erosion. Uplift of the Black Hills and the Central Texas Uplift began as the continental interior was raised and the last Cretaceous sea was displaced, 65 to 70 million years ago. They stood well above the surrounding plains long before any sediments from the distant Rocky Mountains began to accumulate at their bases. In southern Colorado and northern New Mexico, molten rock invaded the sedimentary layers between 22 and 26 million years ago. The Spanish Peaks were formed at this time from hot magma that domed up the surface layers but did not break through; the magma has since cooled and solidified and has been exposed by erosion. Elsewhere the magma reached the surface, forming volcanoes, fissures, and basalt flows. A great thickness of basalt flows accumulated at Raton Mesa and Mesa de Maya between 8 and 2 million years ago. Volcanism has continued intermittently, and the huge cinder cone of Capulin Mountain was created by explosive eruption only 10,000 to 4,000 years ago. Most of these volcanic masses were formed before major downcutting by the streams began. Other igneous intrusions and volcanic areas in the northern Great Plains similarly were formed before the streams were incised.

To examine the origin of the present landscape and of the landforms typical of the various sections of the Great Plains, it is convenient to begin with the Black Hills, the Central Texas Uplift, and the Raton section simply because they were formed first—they existed before the other sections were outlined.

BLACK HILLS

The Black Hills is a huge, elliptically domed area in northwestern South Dakota and northeastern Wyoming, about 125 miles long and 65 miles wide (fig. 10). Rapid City, S. Dak., is on the Missouri Plateau at the east edge of the Black Hills. Uplift caused erosion to remove the overlying cover of marine sedimentary rocks and expose the granite and metamorphic rocks that form the core of the dome. The peaks of the central part of the Black Hills presently are 3,000 to 4,000 feet above the surrounding plains. Harney Peak, with an altitude of 7,242 feet, is the highest point in South Dakota. These central spires and peaks all are carved from granite and other igneous and metamorphic rocks that form the core of the uplift. The heads of four of our great Presidents are sculpted from this granite at Mount Rushmore National Memorial. Joints in the rocks have controlled weathering processes that influenced the final shaping of many of these landforms. Closely spaced joints have produced the spires of the Needles area, and widely spaced joints have produced the rounded forms of granite that are seen near Sylvan Lake (fig. 11).

Marine sedimentary rocks surrounding the old core rocks form well-defined belts. Lying against the old core rocks and completely surrounding them are Paleozoic limestones that form the Limestone Plateau (fig. 10). These tilted layers have steep erosional scarps facing the central part of the Black Hills. Wind Cave and Jewel Cave were produced by ground water dissolving these limestones along joints. These caves are sufficiently impressive to be designated as a national park and a national monument, respectively. Encircling the Limestone Plateau is a continuous valley cut in soft Triassic shale. This valley has been called “the Racetrack,” because of its continuity, and the Red Valley, because of its color. Surrounding the Red Valley is an outer hogback ridge formed by the tilted layers of the Dakota Sandstone, which are quite hard and resistant to erosion. Streams that flow from the central part of the Black Hills pass through the Dakota hogback in narrow gaps.

Dakota Sandstone hogback Limestone plateau Belle Fourche River Spearfish Bear Butte Sundance Red Valley Rapid City Red Valley Hot Springs Cheyenne River Edgemont Mt. Rushmore National Monument Jewel Cave National Monument Wind Cave National Park

The Black Hills, then, is an uplifted area that has been carved deeply but differentially by streams to produce its major outlines. Those outlines have been modified mainly by weathering of the ancient core rocks and solution of the limestone of the Limestone Plateau.

CENTRAL TEXAS UPLIFT

The domed rocks of the Central Texas Uplift form a topography different from that of the Black Hills. Erosion of a broad, uplifted dome here has exposed a core of old granites, gneisses, and schists, as in the Black Hills, but in the Central Texas Uplift, erosion has produced a topographic basin, rather than high peaks and spires, on the old rocks of the central area. A low plateau surface dissected into rounded ridges and narrow valleys slopes gently eastward from the edge of the central area to an escarpment at the Balcones fault zone, which determines the eastern edge of the Great Plains here. Northwest of the central basin the Colorado River flows in a broad lowland about 100 miles long, but the northern edge of the uplift, forming a divide between the Brazos and the Colorado Rivers, is a series of mesas formed of more resistant sandstone and limestone.

The cutting action of streams, modified or controlled in part by differences in hardness of the rock layers, has been responsible for the landforms of the Central Texas Uplift. Weathering of the old core rocks has softened them sufficiently to permit deeper erosion of the central area, and solution of limestone by ground water has formed such features as Longhorn Caverns, 11 miles southwest of Burnet, Tex.

RATON SECTION

Volcanism characterizes the Raton section. The volcanic rocks, which form peaks, mesas, and cones, have armored the older sedimentary rocks and protected them from the erosion that has cut deeply into the adjoining Colorado Piedmont to the north and Pecos Valley to the south. The south edge of the Raton section is marked by a spectacular south-facing escarpment cut on the nearly flat-lying Dakota Sandstone. This escarpment is the Canadian escarpment, north of the Canadian River. Northward for about 100 miles, the landscape is that of a nearly flat plateau cut on Cretaceous rock surmounted here and there by young volcanic vents, cones, and lava fields. Capulin Mountain is a cinder cone only 10,000 to 4,000 years old (fig. 12). Near the New Mexico-Colorado border, huge piles of lava were erupted 8 to 2 million years ago onto an older, higher surface on top of either the Ogallala Formation of Miocene age or the Poison Canyon Formation of Paleocene age. (See table 1.) These lava flows formed a resistant cap, which protected the underlying rock from erosion while all the surrounding rock washed away. The result is the high, flat-topped mesas, such as Raton Mesa and Mesa de Maya (fig. 13), that now form the divide between the Arkansas and Canadian Rivers. At Fishers Peak, on the west end of Raton Mesa, about 800 feet of basalt flows rest on the Poison Canyon Formation at about 8,800 feet in altitude. Farther east, on Mesa de Maya, about 400 feet of basalt flows overlie the Ogallala Formation at altitudes ranging from about 6,500 feet at the west end to about 5,200 feet at the east end, some 35 miles to the east. The Ogallala here rests on Cretaceous Dakota Sandstone and Purgatoire Formation, for the Poison Canyon Formation was removed by erosion along the crest of a local uplift before the Ogallala was deposited.

East of the belt of upturned sedimentary layers that form the hogback ridges at the front of the Rocky Mountains, the layered rocks in the Raton Basin have been intruded in many places by igneous bodies, the two largest of which form the Spanish Peaks (fig. 14), southwest of Walsenburg, Colo. These two peaks are formed by igneous bodies that were intruded 26 to 22 million years ago and have since been exposed by removal of the overlying sedimentary rock layers by erosion. Radiating from the Spanish Peaks are hundreds of dikes, nearly vertical slabs of igneous rock that filled fractures radiating from the centers of intrusion. Erosion of the sedimentary layers has left many of these dikes as conspicuous vertical walls of igneous rock that project high above the surrounding land surface. Some of these dikes north of Trinidad, Colo. extend eastward for about 25 miles, almost to the Purgatoire River.

The northern boundary of the Raton section is placed somewhat indefinitely at the northern limit of the area injected by igneous dikes. The eastern boundary of the Raton section is at the eastern margin of the lavas of Mesa de Maya and adjoining mesas, where lava-capped outliers of Ogallala Formation are separated from the Ogallala of the High Plains only by the canyon of Carrizo Creek.

HIGH PLAINS

At the end of Ogallala deposition, some 5 million years ago, the Great Plains, with the exception of the uplifted and the volcanic areas, was a vast, gently sloping plain that extended from the mountain front eastward to beyond the present Missouri River in some places. Regional uplift of the western part of the continent forced the streams to cut downward; land near the mountains was stripped away by the Missouri, the Platte, the Arkansas, and the Pecos Rivers, and the eastern border of the plains was gnawed away by lesser streams. A large central area of the plain is preserved, however, essentially untouched and unaffected by the streams, as a little-modified remnant of the depositional surface of 5 million years ago. This Ogallala-capped preserved remnant of that upraised surface is the High Plains. In only one place does that old surface still extend to the mountains—at the so-called “Gangplank” west of Cheyenne, Wyo. (fig. 15). In places, as at Scotts Bluff National Monument, Nebr. (fig. 16), small fragments of this surface have been isolated from the High Plains by erosion and now stand above the surrounding area as buttes.

The High Plains extends southward from the Pine Ridge escarpment, near the South Dakota-Nebraska border (fig. 3), to the Edwards Plateau in Texas. The Platte, the Arkansas, and the Canadian Rivers have cut through the High Plains. That part of the High Plains south of the Canadian River is called the Southern High Plains, or the Llano Estacado (staked plain). The origin of this name is uncertain, but it has been suggested that the term Llano Estacado was applied by early travelers because this part of the High Plains is so nearly flat and devoid of landmarks that it was necessary for those pioneers to set lines of stakes to permit them to retrace their routes.

The Llano Estacado is bounded on the west by the Mescalero escarpment (fig. 4) and on the east by the Caprock escarpment. The southern margin with the Edwards Plateau is less well defined, but King Mountain, north of McCamey, Tex., is a scarp-bounded southern promontory of the High Plains. The remarkably flat surface of the Llano Estacado is abundantly pitted by sinks and depressions in the surface of the Ogallala Formation; these were formed by solution of the limestone by rainwater and blowing away or deflation by wind of the remaining insoluble particles. Many of these solution-deflation depressions are aligned like strings of beads, suggesting that their location is controlled by some kind of underlying structure, such as intersections of joints in the Ogallala Formation.

The solution-deflation depressions are less abundant north of the Canadian River, but occur on the High Plains surface northward to the Arkansas River and along the eastern part of the High Plains north of the Arkansas to the South Fork of the Republican River.

Covering much of the northern High Plains, however, are sand dunes and windblown silt deposits (loess) that mantle the Ogallala Formation and conceal any solution-deflation depressions that might have formed. The Nebraska Sand Hills (fig. 17), the largest area of sand dunes in the western hemisphere, is a huge area of stabilized sand dunes that extends from the White River in South Dakota southward beyond the Platte River almost to the Republican River in western Nebraska but only to the Loup River in the northeast part of the High Plains (fig. 18). Loess covers the western High Plains southward from the sand dunes almost to the Arkansas River, and to the South Fork of the Republican in the eastern part. This extensive cover of loess has created a fertile land that makes it an important part of America’s wheatlands (fig. 19).

Other, smaller areas of sand dunes lie south of the Arkansas River valley. The only large areas of sand dunes on the Llano Estacado, or Southern High Plains, are along the southwestern margin near Monahans, southwest of Odessa, Tex.

Oil and gas are present in the Paleozoic rocks that underlie the High Plains at depth. Gas fields are ubiquitous in much of the eastern part of the High Plains between the Arkansas and Canadian Rivers. Just south of the Canadian River, at the northeast corner of the Southern High Plains, a huge oil and gas field has been developed near Pampa, Tex. Oil and gas fields also are abundant in the southwestern part of the Southern High Plains, south of Littlefield, Tex.

WYOMING Badlands National Monument Missouri River Valley JAMES RIVER LOBE MINNESOTA IOWA SOUTH DAKOTA NEBRASKA Rosebud Valentine DES MOINES LOBE NEBRASKA Ashby SANDHILLS Platte River Valley IOWA MISSOURI NEBRASKA KANSAS COLORADO Muscotah TOPEKA EXPLANATION Transverse dunes Longitudinal dunes Wind-blown sand Loess thickness (in feet)

The surface of the High Plains, then, has been little modified by streams since the end of Ogallala deposition. It has been raised by regional uplift and pitted by solution and deflation, and large parts of it have been covered by wind-blown sand and silt. It has been drilled for oil and gas and extensively farmed, but it is still a geological rarity—a preserved land surface that is 5 million years old.

MISSOURI PLATEAU

Beginning about 5 million years ago, regional uplift of the western part of the continent forced streams, which for 30 million years had been depositing sediment nearly continuously on the Great Plains, to change their behavior and begin to cut into the layers of sediment they so long had been depositing. The predecessor of the Missouri River ate headward into the northern Great Plains and developed a tributary system that excavated deeply into the accumulated deposits near the mountain front and carried away huge volumes of sediment from the Great Plains to Hudson Bay. By 2 million years ago, the streams had cut downward to within a few hundred feet of their present level. This region that has been so thoroughly dissected by the Missouri River and its tributaries is called the Missouri Plateau.