Pedestal Rock in the Killdeer Mountains, circa 1911. The Killdeer Mountains consist of two mesas that rise 700 feet above the surrounding countryside in northern Dunn County. The rocks that cap the mountain, shown here, consist of tuffaceous sandstone, siltstone, and carbonates of the Arikaree Formation.
People like to draw lines on maps; it helps them to describe and know their world. Politicians scribe political boundaries that guide the development of government; hydrologists draw watershed boundaries that define individual drainage basins; entrepreneurs chart transitory lines on maps that guide their market development. Among the lines that geologists and geographers use are those that define physiographic regions. North Dakota has been so divided into six major physiographic regions, each defined by a suite of characteristic landforms that serve to differentiate it from its neighbors.
One of the best ways to "see" these
physiographic regions is by using a shaded-relief map. Such a map
vividly shows the dramatic differences in the landscape.
Certainly the new shaded-relief map of North Dakota (see pages
14-15) dispels the myth of the uniform, monotonously flat
countryside that many outsiders envision, and that sometimes even
locals, traveling at highway speeds, experience. The great, wide
open countryside at times makes it difficult to appreciate the
diversity of landscapes within one's view. When compressed onto a
page-size map, however, major topographic features are easy to
The Coteau Slope of eastern Burleigh County. This rolling to hilly area is characterized by both erosional and glacial landforms. (R.F. Biek photo)
The boundaries of these physiographic regions are not arbitrary,
not capriciously drawn like the misleading names hung on some suburban
streets. Each line reflects an important underlying geologic feature.
The Missouri Escarpment, for example, marks a prominent slope that
advancing glaciers were forced to push up and over, eventually leaving
behind a characteristic hummocky topography and innumerable prairie
potholes. The remarkably flat floor of the Red River Valley mirrors
the surface of the Ice Age lake, glacial Lake Agassiz, in which it
formed. Beaches and wave-cut scarps now mark the lake's former shorelines
and separate the valley from the glaciated plains to the west. Understanding
the state's physiography is very much understanding an outline of
the state's geologic history.
Map identifying the major physiographic regions in North Dakota. Even though landforms of considerable variation occur within each region, overall internal similarities make it possible to generalize about the geomorphic processes that operated to shape each region.
The Little Missouri Badlands are carved from 55-million-year-old river, lake, and swamp sediments of the Fort Union Group. The term "badlands" attests to the intricate, deeply dissected nature of the land, with gullies, buttes, and a maze of short, steep ridges that make travel through such areas difficult. The Sioux Indians knew the badlands as "mako sica" ("land bad"), while early French explorers translated this to "les mauvais terres a' traverser" ("bad land to travel across"). (R.F.Biek photo)
North Dakota lies within the Interior Plains,
that vast region stretching from the Rocky Mountains to the
Appalachians. In North Dakota, the Interior Plains are divided
into two major physiographic provinces by the Missouri
Escarpment. To the north and east of the escarpment lies the
Central Lowlands Province, characterized by its glacially
smoothed landscape. To the south and west, the Great Plains
Province rises gradually westward toward the Rocky Mountains. As
we shall see, the Missouri Escarpment, while prominent and
readily defined along most of its length, does not neatly
separate these two major physiographic divisions, but, as with
most things natural, the boundary is marked by a transition zone,
here called the Missouri Coteau.
The Great Plains Province is divided into the Missouri Plateau (or Missouri Slope Upland), Little Missouri Badlands, Coteau Slope, and Missouri Coteau. The Great Plains Province thus contains both glaciated and non-glaciated regions. Southwest of the Missouri River, the broad valleys, hills, and buttes of the Missouri Plateau are largely the result of erosion of flat-lying beds of sandstone, siltstone, claystone, and lignite. These sediments belong primarily to the Paleocene-age Fort Union Group and were deposited by ancient rivers flowing away from the rising Rocky Mountains between about 65 to 55 million years ago. From about 10 to 5 million years ago, streams began eroding the sediments that had so long ago been deposited, dissecting the plateau with a series of rivers flowing northeast to Hudson Bay. The modern landscape over most of southwestern North Dakota thus formed over an exceptionally long period of time, unlike the much more recent topography of the glaciated portion of the state.
The badlands of southwestern North Dakota are carved into an astonishing variety of unusually shaped landforms. Shown here is the lower Sentinel Butte Formation in the South Unit of Theodore Roosevelt National Park. (North Dakota Tourism Department photo)
The Little Missouri Badlands are carved into
strata of the Missouri Plateau. The badlands are a rugged, deeply
eroded area along the Little Missouri River that stretches from
Bowman County north to the confluence with the Missouri River.
The Little Missouri River began to carve the badlands about
600,000 years ago during Pleistocene time (the "Ice
Age") when the river was diverted by glaciers from its
northerly route into Canada. As a result of this diversion, the
Little Missouri River was forced to flow eastward over a shorter,
steeper route, thus beginning a cycle of vigorous erosion that
Typical prairie pothole, formed by the melting of sediment-laden glacial ice. As the ice melted, the sediment slumped and slid into low areas, creating small hills. The small lakes, or potholes, generally occupy areas where the ice persisted the longest, preventing them from becoming filled with sediment. (J.P. Bluemle photo)
The spectacular variety of landforms found in the
Missouri Plateau and Little Missouri Badlands results primarily
from the differences in resistance to erosion among Fort Union
Group strata. Buttes, for example, form when easily eroded
sediments are protected by a hard layer of sandstone or
limestone. Where beds of lignite have caught fire and burned,
adjacent sediments are baked and fused into a natural brick-like
material called clinker. The bright red clinker also shields
underlying sediments from erosion. In other places, mineralized
groundwater circulated through the sediments, forming flint,
petrified wood, silcrete, and concretions and nodules of all
shapes and sizes, all of which, being harder than the enclosing
sediments, resist erosion and so accumulate at the surface.
Although part of the Missouri Plateau south and west of the Missouri River was glaciated during the Pleistocene, in most places the only visible evidence of glaciation is an occasional erratic boulder or thin patch of glacial sediment. The glaciations that affected these areas were early ones that occurred long before the glaciations whose deposits and landforms are so evident in the eastern and northern parts of the state. Presumably, most of the evidence of these early glaciations - thick glacial sediments and glacial landforms - was removed by erosion over the past several hundred thousand years.
Boulder-covered surface of the Missouri Escarpment. Slopewash has removed some of the finer grained sediment, leaving behind a lag deposit of boulders. (J.P. Bluemle photo)
North and east of the Missouri River, the Great
Plains Province differs markedly from that south and west of the
river, principally because evidence of glaciation is so fresh and
clear. Here it is divided into the Coteau Slope, a rolling to
hilly land with both erosional and glacial landforms, and the
Missouri Coteau, a hummocky landscape characterized by
innumerable prairie potholes. Because of its distinct glacial
character, many geologists consider the Missouri Coteau to be
part of the Central Lowlands Province. The Missouri Coteau does
in fact share features of both the Central Lowlands and the Great
Plains provinces and so perhaps it would be best to think of the
Coteau as a transition zone between the two regions.
Aerial view of typical collapsed glacial topography in Nelson County, North Dakota. Note hummocky surface with numerous prairie potholes. (J.P. Bluemle photo)
The Missouri Coteau trends through the state,
parallel to and east of the Missouri River. It consists of
hummocky topography - thus the Canadian French coteau meaning
"little hill" - characterized by unintegrated drainage
(meaning that ponds and sloughs are not connected to one another
and no streams flow through the area). The landscape of the
Missouri Coteau formed because glaciers were forced to advance up
a steep escarpment before they flowed onto the uplands. As
glaciers advanced over the escarpment, sediment from the base of
the glacier was forced up to the surface. When the climate
moderated and the glaciers stagnated, sediment melting out of the
ice accumulated at the surface, insulating the ice so that it
took several thousand years to melt completely. As it melted,
sediment slumped and slid forming the hummocky topography.
Prairie potholes are most numerous where large-scale glacial
stagnation processes dominated.
The topography of the Turtle Mountains and the Prairie Coteau formed in a similar manner to that of the Missouri Coteau. They are erosional bedrock outliers draped with glacial sediments. They, too, are characterized by a hilly, irregular topography with many small ponds and lakes.
The Coteau Slope is a rolling to hilly region that contains both glacial and erosional landforms. Unlike the Coteau that bounds its eastern margin, drainage within the Coteau Slope is generally well developed, so that there are comparatively few potholes. The north and east margin of the Missouri Coteau is marked by the Missouri Escarpment. The escarpment is a prominent feature along most of its length, in places rising 600 feet above the comparatively level terrain of the Glaciated Plains.
Devils Heart Butte, located south of Devils Lake in Benson County, North Dakota. This 175-foot-high hill occurs in association with intensely ice thrust topography. It is composed of sand and gravel and may have formed a "veblin," a hydrodynamic blowout feature, when high-pressure groundwater flowed to the surface during glacial thrusting. (J.P. Bluemle photo)
|The Glaciated Plains is a rolling, glaciated landscape also known as the drift prairie. Much of the region is very gently sloping, in contrast to the deeply dissected Missouri Plateau. In other places, the ice shoved and thrust large masses of rock and sediment, forming ice-thrust hills near the ice margin. In still other areas, loose accumulations of rock and sediment piled up at the edge of a glacier, resulting in areas of especially hilly land called end moraine. Even where the landscape harbors only the gentlest of swales, interesting landforms can be seen. Washboard moraines, for example, create a corrugated landscape, each marking a small ice thrust zone. Shorelines of glacial lakes, large channels carved by catastrophic floods of glacial meltwater, sinuous ridges of sand and gravel called eskers, and a variety of other glacial landforms are found on the Glaciated Plains. The Glaciated Plains contain some of the most remarkable glacial features to be found anywhere. Most of these landforms were shaped very late in the Pleistocene by the most recent glaciation, known as the Wisconsinan, which spanned the time from about 70,000 until 10,000 years ago. Even though earlier glaciations played an important role in shaping the landscape, the changes they wrought are largely concealed, buried beneath more recent glacial deposits.|
The Red River of the North. The Red River meanders across the flat floor of glacial Lake Agassiz. (North Dakota Tourism Department photo)
The Sheyenne River valley, one of many, deep, steep-walled glacial outburst channels in the Glaciated Plains. Channels like this, now occupied by comparatively small rivers and intermittent streams, were carved at the close of the Ice Age by catastrophic floods of glacial water. (J.P. Bluemle photo)