Regions 1 and 2: The Central Lowland and Great Plains

The two physiographic regions of the Central Lowland and Great Plains are combined in this section due to their geological continuity. The Central Lowland, an area of low terrain that extends like a saucer with gently rising rims, stretches to meet the Great Plains on its western border in the eastern Dakotas and eastern Nebraska. In general, surface deposits in these two regions are composed of Quaternary glacial tills and outwash in the northernmost and easternmost plains, and Mesozoic-Cenozoic sediments in the western plains. Outcrops of older material are usually exposed by stream erosion, dissected terrain, or quarries. Erosional processes from the Missouri, Yellowstone, Little Missouri, Cheyenne, Niobrara, and Platte river systems dominate the area’s active geology.

The Great Plains and Central Lowland are underlain by a basement of igneous and metamorphic Precambrian rocks, some of which are up to 2.6 billion years old. These rocks are, for the most part, buried and inaccessible, with the exception of the Black Hills in southwestern South Dakota and the Sioux Arch in southeastern South Dakota.

The Sioux Arch area contains Proterozoic Sioux Quartzite, a formation of pink and red orthoquartzite with cross-bedding, ripples, and mudcracks. It consists largely of conglomerates formed from stream deposits, sandstones from braided streams and alluvial plains, and red to purple mudstones from tidal and lagoonal deposits. These materials, eroded from Archean granites, sandstones, and iron formations, were deposited between 1.8 and 1.6 billion years ago before being subjected to mild metamorphism.

Although the Sioux Quartzite is largely overlain by Cretaceous rocks and Pleistocene glacial materials, it appears in small outcrops in southeastern South Dakota and adjacent Minnesota, and is exposed in abundance at Sioux Falls Park along the Big Sioux River (Figure 2.2). The quartzite is quarried for building and decorative material (Figure 2.3), and the mudstones are also known as “pipestone” since Native Americans quarried them for pipes and carvings (Figure 2.4).

Figure 2.2: Sioux Falls Park, Sioux Falls, South Dakota.

Figure 2.3: Sioux Quartzite was used to construct the Federal Building in Sioux Falls, South Dakota.

Figure 2.4: Native American pipe bowl, carved from pipestone into the shape of an owl.

The most dramatic outcrops of Precambrian rocks within the Great Plains and Central Lowland are located in South Dakota’s Black Hills. The Black Hills are the easternmost outlier of the Cordilleran system, uplifted during the Laramide Orogeny between 68 and 65 million years ago. The range is cored by a complex set of 3.5- to 2.5-billion-year-old Archean rocks that were later deformed and metamorphosed into various schists and gneisses accompanied by the intrusion of granitic rocks. At the very center of the uplift is the notable 1.7-billion-year-old Harney Peak granite batholith from which Mt. Rushmore is carved (Figure 2.5). Related pegmatites known for a great variety of spectacular minerals and crystals are also found here.

Figure 2.5: Mt. Rushmore, carved from the Harney Peak granite.

Thick sequences of Paleozoic and Cenozoic sedimentary rocks cover the basement beneath the Great Plains. Layers of limestone and shale were deposited when shallow seas repeatedly flooded the area, while sandstones accumulated from sandy beaches were left behind as the seas retreated. These sedimentary layers are largely undeformed except where they have been pushed up and exposed by uplift in the Black Hills. Here, extensive cave systems formed in the Mississippian-aged Madison limestone (locally known as Pahasapa limestone) after the layers were uplifted and subjected to surface erosion (Figure 2.6). The delicate formations found in these caves today, called speleothems, are mineral deposits that formed in more recent times.

Figure 2.6: Jewel Cave in the Black Hills of South Dakota. The cave was formed as acid-rich water gradually dissolved layers of limestone that had been cracked by the uplift of the Black Hills around 60 million years ago.

In easternmost Nebraska, a small area of Carboniferous strata is exposed at the surface thanks to erosion from the Mississippi River. The dark shales and coal beds in this area originate from a swampy shoreline and oxygen-poor continental shelf. Here, rivers flowing from the east deposited sediments eroded from the Appalachian Mountains. A band of Permian bedrock is also exposed in the southeastern portion of Nebraska, deposited there as sea levels moved back and forth across the state during the late Paleozoic.

Throughout the Mesozoic, shallow seas periodically covered much of North America’s interior. The sedimentary deposits resulting from the water’s advance and retreat became the limestones, shales, and sandstones that are now near the surface and actually outcrop in many areas of the Great Plains. For example, Triassic and Jurassic deposits of red silts and clays surround the Black Hills in a ring, providing evidence of an ancient arid coastal plain and intertidal mudflats. These red stones and interbedded layers of gypsum are part of a geological formation called the Spearfish Formation, which extends from the Dakotas into Montana, Wyoming, and Nebraska. The Belle Fourche River, which flows from Wyoming to South Dakota, cuts through and exposes these layers (Figure 2.7).

Figure 2.7: The Spearfish Formation is cut by the Bell Fourche River near Devils Tower National Monument, Wyoming.

During the Cretaceous period, the interior of North America was downwarped by tectonic processes associated with the subduction of oceanic lithosphere along the western edge of North America. As the Laramide and Sevier orogenies occurred to the west, the North American interior was flooded by a particularly vast inland sea called the Western Interior Seaway (Figure 2.8). Episodes of transgression and regression deposited thousands of feet of marine and terrestrial sedimentary rock across the Great Plains and Central Lowland. As the Cretaceous drew to a close, mountain building progressed eastward, and the vast inland sea receded for the final time. The pattern of sedimentation transitioned from marine, to near shore, and finally to on-land gravels, sands, and muds deposited by the action of streams and rivers flowing eastward from the elevated Rockies. These continental deposits covered the entire Great Plains progressively from north to south.

Figure 2.8: The Western Interior Seaway.

Why are there different sedimentary rocks in different environments?

Most sedimentary rock deposited in underwater settings originated from material eroded on land and washed down streams or rivers before settling to the bottom of a body of water. Intuitively, the faster the water is moving, the larger the sediments it may carry. As the water slows down, the size of sediments it can carry decreases. Furthermore, the farther the grains of sediment are carried, the more rounded they become as they are tumbled against each other. In this way, rivers emptying into a sea are effectively able to sort sediment. Near the mouth of the river, the water is still relatively high-energy, dropping only the largest pieces; farther from the shore, the dropped particles get smaller. Therefore, conglomerates and sandstones are interpreted to have been deposited on or near the shore, siltstone farther from the shore, and shale in deep water quite far from shore where currents are slow enough that even very tiny particles may settle.

Increased distance from shore and water depth can also reduce the presence of oxygen in the water, causing organic material to decompose less completely. This causes darker, carbon-rich rocks (including some that contain exploitable fossil fuels) to form in these areas. Limestone is made primarily of calcium carbonate, the components of which are dissolved in the water. Living creatures, like coral and foraminifera, take those components out of the water to make calcium carbonate shells, which, after the creatures die, accumulate to become limestone. These shelled creatures tend to fare better in clear water, so limestone usually forms far from other sources of sediment. While this process happens over much of the seafloor, if more than 50% of the sediment being deposited is from another source, the rock that forms is, by definition, not limestone.

Tectonic activity associated with the Laramide Orogeny also generated volcanism and igneous intrusions near the area of mountain building. The famous Devils Tower, an exposed igneous intrusion that rises 386 meters (1267 feet) above the surrounding terrain, lies in Wyoming just west of the Black Hills (Figure 2.9). Devils Tower is composed of phonolite, a gray or greenish gray igneous rock containing conspicuous crystals of white feldspar. This igneous rock exhibits spectacular columnar jointing, indicating that it cooled quickly at a shallow depth. A popular interpretation for the formation of this landmark classifies it as a solidified volcanic neck, but alternate interpretations peg it as a laccolith or other shallow intrusive body. Just 6 kilometers (3.5 miles) to the northwest of Devils Tower lies a set of four summits, the Missouri Buttes, which are also composed of jointed phonolite of the same age (Figure 2.10). A similar landform in Montana, Snake Butte, is also the result of an igneous intrusion; it is composed of a coarse-grained igneous rock called syenite, and it also exhibits columnar jointing (Figure 2.11). Syenite is particularly durable, and was an important source of material used to build the Ft. Peck Dam in the 1930s.

Figure 2.9: Devils Tower, a large intrusive igneous rock formation with well-developed columnar jointing, in Crook County, Wyoming.

Columnar Jointing

As a lava flow cools, it contracts, and the resulting force may cause the rock to crack. These cracks continue down to the bottom of the flow, resulting in five- or six-sided columns. Columnar joints are not restricted to basalt flows and can form in ashflow tuffs as well as shallow intrusions. The columns are generally vertical, but may also be slightly curved.

Figure 2.10: Devils Tower and the Missouri Buttes at sunrise.

Volcanic eruptions in the Rockies during the Neogene and Paleogene generated ash that was carried eastward by the prevailing winds, and often fell across the Great Plains in thick layers. The Ashfall Fossil Beds in northeastern Nebraska are an example of one such location, formed after a dense volcanic ash fall that occurred in the late Miocene. Sentinel Butte in North Dakota also contains a widespread ash and bentonite deposit that is up to 8 meters (25 feet) thick in some areas (Figure 2.12).

Figure 2.11: Columnar jointing at Snake Butte, an exposed igneous sill located on the Ft. Belknap Reservation in Montana.

Figure 2.12: The Sentinel Butte Formation, a Paleocene ash deposit in the Little Missouri Badlands of North Dakota.

During the Cenozoic, many sediments were deposited in terrestrial environments such as lakes, rivers, and floodplains. These deposits cover the region’s Cretaceous rocks in two large areas. The first, comprising mostly Paleocene sediments, is located in the northern Great Plains of Montana, Wyoming, and the Dakotas. The other area includes a large tract of late Paleocene to Neogene strata that has escaped much erosional loss, and constitutes the High Plains subdivision of the Great Plains between Nebraska and Texas. The sandstones in the High Plains Ogallala Formation house the famous Ogallala or High Plains Aquifer. Water in the Ogallala Aquifer, stored since Quaternary times, is now being withdrawn by extensive agricultural development at rates exceeding recharge in the modern climate regime.

Rivers flowing eastward out of the Rocky Mountains since the early Cenozoic have eroded and carried sediment towards the plains. The process was intensified by the successive accumulation and melting of mountain glaciers and ice caps over many of the mountain ranges in the Rockies. These rivers, continually cutting into and removing earlier sedimentary cover, have thereby created much of the scenery and spectacular rock outcrops found in the Great Plains. Some examples include the Upper Missouri Breaks National Monument in central Montana (Figure 2.13), Badlands National Park in southwestern South Dakota (Figure 2.14), and the Scotts Bluff National Monument in western Nebraska (Figure 2.15). Many of these sculpted badland areas also contain abundant concretions and nodules, hard rounded bodies of rock formed by the precipitation of dissolved minerals, and later exposed by erosion. For example, large spherical sandstone concretions called “cannonballs” are common in the Sentinel Butte Formation of western North Dakota (Figure 2.16).

The Quaternary deposits of the Great Plains and Central Lowland are primarily related to glacial processes. During the ice age, the Laurentide Ice Sheet advanced several times in four main pulses and covered northern Montana and most of the Dakotas, and also penetrated into Nebraska and Kansas. The advancing ice sheet scoured and abraded the bedrock beneath it, breaking it down from huge boulders into fine dust, called rock flour. When the glaciers retreated, till and outwash were carried in meltwater and deposited in lakes or by streams. Rock flour and sand was picked up by the wind and blown for many kilometers (miles) until it settled into thick layers of loess (Figure 2.17). The Sandhills of Nebraska are perhaps the best-known example of wind-transported glacial sediments in the Great Plains.

Figure 2.13: The Upper Missouri Breaks in central Montana are composed of Mesozoic and Cenozoic shales, sandstones, and volcanic materials.

Figure 2.14: The Brule Formation, exposed in Badlands National Park, is a sequence of fine-grained mudstones, claystones, and siltstones interbedded with freshwater carbonate rock, volcanic ash, and sandstone. These sediments were deposited during the Oligocene, 34-30 million years ago.

Figure 2.15: Scotts Bluff exposes 225 meters (740 feet) of Paleogene-Neogene terrestrial sediments, including sandstone, limestone, and volcanic material.

Figure 2.16: Cannonball concretions in the Sentinel Butte Formation, Theodore Roosevelt National Park, North Dakota.

Figure 2.17: Loess deposits in the central US.