Several years ago, billboards were posted around North Dakota in an effort to entertain and catch motorists’ attention. One of them, outside of Mandan, read “North Dakota Mountain Removal Project Completed.” The billboard referred to the image many people have of North Dakota as a flat and featureless land, but the sign ignored the fact that, within our borders are at least half-a-dozen features bearing the name “mountain.” Was the removal project a failure?
One of our mountainous areas is the Killdeer Mountains in western North Dakota, about 40 miles due north of Dickinson. Another is Turtle Mountain, home of the International Peace Garden on the North Dakota-Manitoba border. (Turtle Mountain is singular, not plural; I’ll explain later). It may seem odd that the Killdeer Mountains, Turtle Mountain, and several other features in North Dakota are called “mountains.” The idea may be related somewhat to scale. When viewed by a person who has recently traveled over eastern North Dakota, the features may be impressive, but I wonder what they might have been named if more of our settlers had come via Wyoming or Montana.
We have several more places in North Dakota that bear the name “mountain.” The town of Mountain in Pembina County was settled by Icelanders in 1873. Mountain is situated on the former shoreline of glacial Lake Agassiz, and the view from there, over the Red River Valley, is impressive. Just north of Mountain, the hilly area along the Pembina River Valley in northeastern North Dakota is sometimes referred to as “Pembina Mountain,” but the term “Pembina Hills” is commonly used as well. The steep escarpment is also referred to as the “Pembina Escarpment” or “Manitoba Escarpment.” Pioneer geologist David Dale Owen, when he traveled through the Red River Valley in 1848, commented on Pembina Mountain thus: it is “in fact no mountain at all, nor yet a hill. It is the terrace of table land – the ancient shore of a great body of water that once filled the Red River Valley.” People have been critical of the kind of mountains we have in North Dakota ever since! (Owen was from Indiana).
Other named “mountains” in North Dakota include Devils Lake Mountain in southeastern Ramsey County, Blue Mountain in western Nelson county, Lookout Mountain in northeastern Eddy County, and the Prophets Mountains in western Sheridan County. All of these features are ice-thrust hills or complexes of ice-thrust topography that stand as high as a few hundred feet above the surrounding areas. Near Medora, in Billings County, we have Tracy Mountain, but we don’t have many “mountains” in southwestern North Dakota – in that area the term “butte” is used more often. Several hundred formally named features called “hills” or “buttes” are found in North Dakota, as well as a few “points” and “ridges.” Many of these are at least as impressive as some of our mountains. We also have many features that fit the formal definition of a mesa, but very few of them have been referred to as mesas. I won’t dwell any longer on the vagaries of naming topographic features. The names don’t necessarily make much sense. We do manage to communicate, at least if we stay close to home. This article will deal mainly with the Killdeer Mountains and I will follow it with an article on Turtle Mountain. Both features are of considerable scenic beauty no matter what you want to call them and both have interesting stories to tell. The Killdeer Mountains
The Killdeer Mountains consist of two large, flat-topped buttes in Dunn County. They cover an area of 115 square miles and rise from 700 to 1,000 feet above the surrounding plains. The entire elevated Killdeer Mountain region is about nine miles long and six miles wide. The highest elevation in the area is 3,314 feet, which is 192 feet lower than the highest point in the state (White Butte). The term “Killdeer” is presumably a translation of a Sioux phrase: “Tah-kah-p-kuty” (the place where they kill the deer).
The caprock on the Killdeer Mountains consists of a 300-foot-thick sequence of siltstone, sandstone and carbonate beds that belong to the Miocene-age Arikaree Formation. One of the most conspicuous, ledge-forming units within the Arikaree Formation is found about 150 feet below the caprock. Known as the “burrowed marker unit” or “wormy marker bed,” it is a sequence of hard, erosion-resistant interbedded siltstone and sandstone with some carbonate lenses (the burrows in the bed were dug by clams living in the sediment before it hardened; the organisms that did the digging were similar to modern “shipworms”). The Arikaree Formation lies on top of the Eocene-age Chadron Formation, a sequence of yellow to green sandy mudstone, clayey sandstone, and pebbly sandstone. The tree-covered, slopes around the flanks of the Killdeer Mountains are mainly landslide topography, consisting of materials that have fallen or slid from higher up. A little farther away, the grassy or farmed, less-hilly areas are underlain mostly by the Golden Valley Formation, a Paleocene to Eocene-age rock unit. The Paleocene Sentinel Butte Formation, which underlies the Golden Valley Formation beneath the Killdeer Mountains, occurs at the surface in a broad area surrounding the Killdeer Mountain upland.
The two main buttes that make up the Killdeer Mountains coincide with areas that were once lakes in which sandy and limy sediments, along with some stream deposits, accumulated during Miocene time. Repeated volcanic eruptions in the Rocky Mountains to the west produced large amounts of ash, which blew eastward, fell to the ground, and washed into the lakes, forming tuffaceous (meaning they contain volcanic ash) sandstones.
A new erosion cycle began about five million years ago, long after the lakes had filled with sediment, and dried up. The relatively hard tuffs and freshwater limestone and sandstone beds that had been deposited in the Miocene lakes were much more resistant to erosion than were the surrounding sediments. Because of their resistance to erosion, these hard materials remained standing above the surrounding area as the softer Golden Valley and Sentinel Butte sediments were eroded away by streams and rivers. The Killdeer Mountains, with their resistant caprock, are the result of that erosion; they are the modern manifestation of ancient lake beds. The topography has undergone a complete reversal; areas that were once low are now high due to their resistance to erosion.
Two sites in the Killdeer Mountains are of particular interest. The Killdeer Mountain Battle State Historic Site is located on the southeast edge of the Killdeer Mountains, seven miles northwest of the town of Killdeer (Section 34, T. 146 N., R. 96 W.). The Battle of the Killdeer Mountains took place on July 28, 1864 when General Sully and 2,200 troops used artillery on 6,000 Teton and Yanktonai Sioux in revenge for the uprising of Santee Sioux in southern Minnesota. Sully decimated the Sioux, killing many of them and destroying their camp and equipment. Less than a year earlier, on September 3, 1863, Sully had accomplished a similar feat at the Battle of Whitestone Hill, where his troops killed, captured or wounded 300 to 400 Sioux Indians. Medicine Hole is, indeed, a hole in the ground, but it is not a cave in the traditional sense because it did not form as most caves do. No solution of carbonates was involved, and there are no stalactites or stalagmites. It is, rather, a crack in the ground, where a large block of material has begun to fall away from the main body of the southern butte of the Killdeer Mountain. Medicine Hole is located on private land and, as I write this, in 2015, the area is not open to public access. Please respect the wishes of the land owner. The Killdeer Mountains support the largest deciduous forest in southwestern North Dakota, except for the forests on the floodplains bordering the major rivers. The Killdeer Mountain forest consists largely of aspen and oak, with some ash, elm, birch, and juniper, along with shrubs such as chokecherry, willow, plum and buffaloberry. The forest is interesting in that it contains species that tend to be found in more boreal settings, areas that may be 200 miles or more to the northeast.
In summary, the Killdeer Mountains are an erosional feature, preserved because of their resistant caprock of tuffaceous sandstone and limestone. Erosion of the area began in late Miocene time, and continued into Pleistocene time, resulting in gravel-covered, flat, sloping surfaces (pediments) around the flanks of the Killdeer Mountain uplands. These gravel deposits, up to ten feet thick, were derived from the sandstone and limestone beds higher up in the Killdeers. The gravels are themselves resistant to further erosion and they help to retard the rate of the ongoing, modern erosion cycle. The current erosion cycle began when the nearby Little Missouri River was diverted by a glacier from its northerly route so that it flowed (flows) eastward to its modern confluence with the Missouri River. As a result of the diversion, and the resulting steeper gradient over which it flows, the Little Missouri River began to erode vigorously, carving the badlands through which the modern river flows today. Although the Killdeer Mountains show no evidence of ever having been glaciated, their modern topography dates largely to the Pleistocene. Old ice wedges can be seen in the pediment gravels in places, testimony to the time when the area was subjected to tundra conditions during one or more of the glacial epochs. There were no glaciers over the Killdeers, but continuous frigid conditions provided a tundra ecosystem. *Frost Wedges: Frost wedges are common, but many areas of patterned ground that have been interpreted to be frost polygons are really dessication cracks developed in silcrete. These are much older than frost wedges, such as this one, which formed in loose materials.
Every summer, even during the coldest part of the Ice Age, some melting took place on a glacier’s surface and along its margin. Melting occurred during each summer season – even more when the climate warmed for periods of several years at a time – 20 or 30 years – periods of time comparable to the kinds of swings we see in North Dakota’s climate today. The farther south the glacier advanced, into more temperate zones, the more the amount of melting challenged the health of the glacier in those areas until a balance was finally struck between 1) the rate at which the glacial was ice advancing, 2) warmer climates to the south, and 3) overall climate warming due to the approaching end of the Ice Age. Gradually, the balance among these three factors shifted farther north (and east) and ice began to disappear in those parts of North Dakota that were glaciated.
The position of the edge of an ice sheet at any given time was determined by the balance between melting and the rate at which the glacier was flowing. While the climate remained cold (at average annual temperatures below freezing), a continental ice mass became thicker and the edge of a glacier advanced. When it warmed a little, perhaps with average temperatures a bit cooler than those we have now, the glacier margin melted back about as fast as new ice could be supplied. Even though the glacier was moving, its edge neither advanced nor receded (but melting was taking place on the glacier surface). Given still warmer conditions, the surface of a glacier melted more rapidly; the ice thinned, and the glacier’s edge melted back faster than new ice was being supplied. Areas the ice had covered gradually became deglaciated.
As the margin of a glacier melted, debris that had been frozen into the ice many miles to the north was freed and deposited on the ground. This “glacial sediment” consisted of a blended sampling of the various kinds of rock and sediment over which the glacier had flowed. Glaciers advancing into North Dakota from the northeast deposited mainly sandy, granite-rich materials they had picked up as they flowed over the Precambrian rocks of northwest Ontario. Ice coming from the northwest brought a mixture of sandy and clayey materials it had accumulated as it flowed over broad expanses of Cretaceous shale and Tertiary sands of southern Alberta and Saskatchewan. Ice advancing straight southward from Manitoba, up the Red River Valley, deposited carbonate-rich sediment it had picked up north of Winnipeg where Paleozoic limestone and dolostone are exposed today. If you travel north of Winnipeg to Stony Mountain or Stonewall, Manitoba, be sure to notice the quarries, now producing some of the same materials that glaciers brought to North Dakota, perhaps 17,000 years ago. Studying the composition of the glacial sediment is one way that geologists can determine the direction from which the ice came, and the kind of land it flowed over.
Eventually, as each glacier melted (North Dakota was probably glaciated between 10 and 20 times during the past three million years), the land gradually became free of ice. No reversal of ice flow is involved when the glacier recedes; I emphasize that “retreat” of a glacier refers to the melting of the ice. Three different kinds of ice wasting occurred, at different times and places in North Dakota. The first occurred when the glacier margin may have been far to the south, in Iowa and South Dakota. The result of melting, perhaps over a period of relative warmth of hundreds or thousands of years, resulted in the loss of much of the ice mass off the glacier’s surface. The “view from above” in North Dakota would not have changed much – everything was still all ice – but the thickness of glacial ice covering the land was diminished in thickness by hundreds or thousands of feet, even before the glacier margin had receded into North Dakota.
The second way a glacier wasted (at least from our North Dakota perspective) was when the ice margin was nearby. When that happened, wasting involved frequent change in the position of the edge of the active glacier. As a glacier melted, and after it had become thinner, its active margin gradually receded because the volume of ice arriving was insufficient to replace the ice lost at the edge due to melting. Shrinkage of this kind caused the ice margin to melt back, sometimes in a step-like fashion, the flow of ice pausing long enough at times for the forward movement of the glacier to deliver piles of sediment (moraines) to the receding ice margin. A year of glacial activity might involve the margin moving forward a short distance during a winter; then, during the following summer, the margin receding a slightly greater distance. During this phase, one of the most important things taking place, at least during summer seasons, was the deposition of large amounts of gravel and sand being deposited in front of the glacier by water flowing from the melting, sediment-laden ice. The net effect of this second phase of glacial melting was deglaciation; land that had been covered by ice saw the light of day again, after about 20,000 years.
The third way a glacier wasted involved large-scale stoppage of ice movement, leaving large parts of the glacier stranded, sometimes over broad, mainly upland areas, detached from the main body of still-actively-flowing ice on surrounding lowlands (the plains surrounding Turtle Mountain, for example). In North Dakota, this was important over upland places like the Missouri Coteau and Turtle Mountain. Areas of “stagnant,” or “dead” ice on the uplands then continued to melt slowly. Landforms resulting from the melting of such stagnant ice are distinctive and much different from those that were constructed during the step-wise retreat of active glacial ice I described earlier.
Much of North Dakota’s modern landscape reflects its latest encounter with glaciers during the Ice Age. While glaciers flowed into and over the state, carrying the pulverized rock and soil debris they had picked up along their routes, they sheared off old bedrock landforms, smeared on new layers of sediment, and built new landforms. They filled old river valleys with sediment at the same time rivers of meltwater were flowing from the glaciers carving new valleys. In some places, the glacial ice forced existing rivers to follow different routes; in other places it completely obliterated and concealed what had been rivers and valleys. Cold winds blowing over sand that had earlier been deposited on floodplains and in lakes built dunes and spread a veneer of silt (loess) over much of the state.
Most of the sediment associated with the action of glaciers of the most-recent glaciation is soft. It is “unconsolidated,” and does not hold together well (you can dig it with a shovel). An exception: earlier glaciers also deposited sediment. Nearly all of this earlier sediment has eroded away, but in those places where we have found it exposed, or drilled into it, it may be cemented. A jackhammer may be more appropriate than a shovel for digging in such cemented deposits. However, the softer, looser materials that form most North Dakota glacial deposits are much more common. Sediments related to glaciation in North Dakota can be grouped into three main types: till, lake sediment, and outwash.
1. Till was deposited directly from the ice, mostly in the form of mud flows, which slumped or flowed into their current position as the ice melted. Till consists of silty, sandy, pebbly clay, as well as cobbles, or even large boulders.
2. Lake sediment is layered material that was deposited in lakes, which formed on and near the glacier. Such sediment consists mainly of layers of fine-grained silt and clay, deposited on lake floors, along with some sand and gravel, which collected as beaches along the shores of lakes , many of which were dammed by glaciers.
3. Outwash consists of material deposited by running water. Some outwash may be cemented into a kind of stony concrete, but most of it is loose sand and gravel that was washed out of the melting glacier (hence the name “outwash”). Outwash was deposited by streams and rivers flowing through meltwater valleys or as broad, often nearly level sheets of sand ahead of a melting glacier.
Where they are present, sediments deposited directly by glaciers, and by wind and water associated with glaciation, form a thick covering on top of much of the preglacial (bedrock) surface. In central North Dakota, in Sheridan County, the glacial sediment is over 700 feet thick in places. Ten miles northwest of Tolley in northwestern Renville County, it is at least 800 feet thick, the thickest I can document in the state. The amount and thickness of glacial sediment can vary considerably over short distances so it is likely that even thicker deposits than I mentioned exist in places. Over much of the glaciated part of the state, the glacial materials average 150 feet thick.
At any given location, the glacial deposits may consist of two or more layers of till, interbedded with lake beds, alluvial sediments or other materials. In some places, soils, which had developed on the surface of an earlier glacial, river, lake, or wind-blown deposit, were buried when a new layer of glacial material was deposited. These old, buried soils (paleosols) were formed during long intervals of weathering and exposure, like the one we are enjoying now. Paleosols are among the best indicators in the geological record for multiple episodes of glaciation during the Ice Age. The characteristics of a paleosol also help us understand the climatic conditions (forest or grassland, wet or dry, cool or warm, etc.) at the time it formed.
The best places to see several multiple layers of glacial deposits in North Dakota are near Riverdale, at the Wolf Creek inlet to Lake Sakakawea in McLean County and in Beulah Bay, about 15 miles north of the town of Beulah in Mercer County. In both locations, two and, in some places, three discrete till units, separated by cemented gravel layers or paleosols, are being eroded by waves along the lake.
Drill-hole data in eastern and southeastern North Dakota provide evidence that at least a half dozen glacial advances have occurred there since the Ice Age began. Parts of southwestern North Dakota were glaciated during some of the earlier glaciations, but (apart from some rare exceptions) glacial landforms are not found there today because they were eroded away long ago. Glacial lake sediments and river gravels containing glacially derived materials can be found as far southwest as Dickinson and near the Killdeer Mountains, and I have tentatively identified patches of hard, cemented till and glacial river gravels near Bowman and Rhame, places usually considered never to have been glaciated.
Modern soils are an important link to our geologic past. Fresh glacial deposits consist of a mixture of materials, and, because their sources are so varied, they provide the combination of nutrients necessary for fertile soil. In the glaciated part of the state, North Dakota’s soils consist of the weathered exterior of materials left by glacial action. In the thousands of years that have elapsed since the ice sheets disappeared, constantly changing climate, physical and chemical weathering, accumulation of prairie and woodland plant litter, development of root systems, and burrowing activity by organisms have all contributed to the transformation of glacial deposits into the rich soils that form the basis for much of our agricultural wealth.
Glaciers in North Dakota: Part One
Glaciers are giant bodies of ice, formed from snow that survives from year to year. Accumulations of snowfall from past years compact into a substance called firn, a recrystallized residue of snow left over from past seasons. During the summers, when temperatures are warm enough for rain instead of snow, the rainfall adds to the mass of a glacier, eventually freezing and becoming part of the glacier. With time and additional snow cover, the whole mass gradually solidifies into hard ice: a glacier.
The color of pure glacial ice, if it is clear and lacking the various rocks and sediments often found in glaciers, is ice-blue. Drop a piece of glacial ice into a glass of warm water and it may literally “explode.” Any air trapped in the ice, thousands of years ago, and pressurized by the great overlying weight of the glacier, escapes with force from the piece of ice as it melts in the glass. By analyzing these trapped pockets of air, scientists can learn what our atmosphere consisted of in the past.
When an ice mass becomes thick enough and heavy enough to flow, it “officially” becomes a true glacier. A glacier flows slowly away from the place where it is thickest. It may flow at a few feet a year although, in some circumstances, the flow rate may be much faster. In the northern hemisphere, glaciers expand mainly southward, away from polar regions because the temperatures to the south are warmer than to the north. A glacier flows most easily when it is warmer and less brittle. It will move much faster at 30 degrees F. than at minus 30 degrees F.
A glacier doesn’t glide placidly over the land. It scratches and grinds the underlying bedrock surface, picking up pieces of rock and soil, dragging them along and using them as tools to scour the ground beneath the ice.
Glaciers don’t normally flow uphill, but they do fill lowlands and overtop them, much like flood water.
Glaciers in mountainous areas, unlike broad ice sheets in places like Greenland and Antarctica, come with a sense of “scale.” The mountain peaks in the distance and the valley walls that hold the glacier help you to orient yourself. But suppose you are standing on a snow-covered, continental-size glacier (dressed in a heavy parka). You see nothing but frozen wasteland, nothing but whiteness. No buildings, no fences, no trees, no landmarks. Only emptiness. No sound but the wind. On a cloudy day, sky and ice blend; making it nearly impossible to distinguish the horizon marking their boundary.
The most-recent major glacial episode in North Dakota is referred to as the Wisconsinan glaciation. It began approximately 90,000 years ago and ended 11,500 years ago, but glacial conditions were not continuous during that entire time. An initial pulse of glaciation (the Early Wisconsinan, 90,000 to 70,000 years ago), was followed by withdrawal of the ice, which was probably complete by 65,000 years ago. Between 65,000 and 35,000 years ago, North Dakota’s climate alternated between combinations of warmer, wetter, cooler, and drier periods, much as it does today. Then, about 30,000 years ago, a second major pulse of glaciation, the Late Wisconsinan, began. The Late Wisconsinan glacier reached its maximum extent between 18,000 and 16,000 years ago when it covered all but the southwestern part of the state. The position of the Missouri River approximates the maximum extent of the Late Wisconsinan glacier. Active glaciers melted completely from the state by 11,500 years ago.
By the time each glacier advanced, the land ahead of it may have become deeply frozen permafrost and, as the ice moved over the frozen rock and soil, it picked up chunks of these materials, incorporating them into the glacier itself. In the areas where it formed, west of Hudson Bay, the materials beneath the thickening ice were mainly crystalline igneous and metamorphic rocks such as granite and gneiss. Farther south, the glaciers flowed over layers of sedimentary rock. Whatever the ice flowed over, it picked up some of it and carried it along.
A moving glacier may be likened to a huge excavation and grading machine that does its job of eroding by plucking and abrasion. Plucking, the more important of the two, is based on a freeze-thaw cycle. The cycle begins when downward pressure melts the ice at the base of a glacier. Water seeps into cracks in the rock beneath the glacier. When the water freezes, the expanding ice plucks rock fragments and incorporates them into the debris near the base of the glacial ice.
After a glacier covers an area for a while, and a considerable thickness of ice lies on the land, the materials beneath a glacier gradually thaw, a combined result of the pressure of the overlying ice and the natural upward flow of heat from the Earth’s interior. The ground surface beneath the North Dakota glaciers was not frozen, and the base of a glacier may have been a muddy mass. This facilitated even more sediment being incorporated into the base of the moving ice.
In some places ground water in the saturated sediments beneath the heavy weight of the glacier built up great pressures due to the weight of the overlying ice.
Besides transforming materials beneath the ice into a mud-like mixture, the water, because it was pressurized, tended to force — squeeze–the sub-glacial sediments upward, into the base of the moving glacier. Sediments beneath the ice were smeared out as they were carried along with the advancing mass of ice.
A glacier seldom behaves like a bulldozer, pushing debris ahead of it. It does, however, incorporate debris as it moves by freezing it onto its base. In whatever way the boulders, gravel, sand, silt, and clay beneath a glacier became part of the moving glacier mass, both ice and sediment flowed forward and the farther the glacier traveled, the more material it accumulates. Glaciers advanced over North Dakota several times, and each time, when they melted, they dropped their entire load of rock and sediment, material gathered from places previously overridden. Some of the material carried by the glacier ended up far from where it had originated. We find rocks in North Dakota that came from northern Saskatchewan, Manitoba, and Ontario. We also find chunks of shale that came from only a few dozen feet away. The sediment a glacier was carrying finally came to rest when the last ice melted.
During the active life of a glacier, every crystal of ice, every boulder, sand grain and fragment of rock within the ice, is moving, slowly making its way away from the center of snow and ice accumulation.
In North Dakota, the movement was generally southward, away from the Keewatin center of ice accumulation west of Hudson Bay. Apart from its overall southward progress, variations in the topography over which the glacier advanced locally affected the direction of flow.
When the glaciers that covered much of North Dakota eventually melted, all of the material they had been carrying was laid down on the land surface where the glacier had been. This included everything from large boulders (erratics) to fine-grained material: sand, silt, and clay. This “glacial sediment,” deposited directly from the melting ice, is known as “till.” Till was deposited as a kind of stony mud that eventually dried out after the glacier melted away. Usually, the till amounted to a few tens of feet of material, but after several glaciations, it might have accrued to several hundred feet: the materials from several glaciations, stacked one on top of another.
Rain and melting snow, wind, frost, and other forces of erosion have carved our badlands into intricate shapes. Since the Little Missouri River began to form the badlands, it has removed an enormous amount of sediment from the area. In the southern part of the badlands, near the river’s headwaters and close to Devils Tower in northeastern Wyoming and adjacent Montana, the river has cut down about 80 feet below the level at which it had been flowing before it was diverted by a glacier farther north. Near Medora, the valley floor is 250 feet lower than the pre-diversion level. Still farther downstream, in the North Unit of Theodore Roosevelt National Park and near the confluence of the Missouri and Little Missouri rivers, and nearer to where the glacier diverted it, the east-trending portion of the Little Missouri River flows at a level that is 650 feet deeper than when it was diverted.
The average rates of erosion in the badlands, assuming they started to form about 640,000 years ago, can be calculated as follows:
Headwaters area in Wyoming: 0.15-inch/100 years;
Medora area: 0.5-inch/100 years;
Confluence area near Mandaree – Missouri and Little Missouri rivers: 1.25 inch/100 years.
These rates may seem tiny but, over time, erosion has removed a huge amount of sediment. Approximately 40 cubic miles of sediment have been eroded and carried away by the Little Missouri River from the area that is now the badlands. Most of that sediment now lies beneath the water of the Gulf of Mexico.
The rates of erosion I’ve noted are long-term averages, but erosion goes on at highly irregular rates. Locally, considering only the past few hundred years, the badlands have undergone four separate periods of erosion and three periods of deposition. Since about 1936, new gullies have been cut to their present depths. It may seem a paradox that, although running water is the main agent of erosion, badlands formation tends to be most intense when water is in short supply. Why? Because erosion tends to be more vigorous during times of drought when the vegetative cover is too sparse to protect the soil from the occasional rain storm or spring snow melt. When precipitation is sufficient for the growth of heavy vegetation, the soil is better protected from severe erosion.
Streams and rivers carry sediment away from the area of the badlands, but most of the actual “on-the-spot” erosion is a result of slopewash. In places where vegetation is sparse, the soil and rock materials are easily weathered, forming loose surfaces that slide downslope easily, slumping and sliding during showers or when the snow cover melts.
The Badlands Landscape
The shapes, sizes, and configurations of the hills, buttes, valleys, and other landforms in the badlands are not entirely happenstance. Differences in hardness of the materials result in differences in resistance to erosion. Nodules and concretions help to shape a landscape ranging from beautiful, to desolate – even grotesque. Hard beds of sandstone or clinker cap many of the small buttes. Variations in permeability (permeability is a measure of the ease with which water can move through porous rock) have similar effects; rain and melted snow soak into the more open and permeable sands, resulting in only minimal erosion. When water flows over the surface of tighter, less permeable sediment, such as clay, it abrades and erodes the material, carrying some of it away. The presence or absence and the character of the vegetation also play important roles in governing the rate of erosion. Grass usually helps to control erosion more effectively than does forest vegetation.
The irregular placement of hard nodules and concretions may result in the development of rock-capped pillars, known as “hoodoos,” mushroom-like shapes perched on stalks of clay. In places, slopes are covered by nodules of siderite (iron carbonate). As they weather out of the surrounding materials, becoming concentrated on the surface, the copper-colored nodules form an erosion-resistant armor, which temporarily slows the rate of erosion. Clinker beds are also much more resistant to erosion than are the softer surrounding beds. We commonly see buttes capped by red clinker beds.
Erosional “pipes” sometimes form in gullies and ravines where surface runoff is focused. “Piping” results where runoff can flow downward into small cracks and joints. Pipes are common in places where surface runoff erodes cavities vertically downward through the soft rock. With time, the initial pathways may widen at depth into caves the size of small rooms. The average depth of vertical pipes is about 10 to 15 feet, but some are much deeper. The tops of pipes may be partially concealed making hiking treacherous. I have seen the bones of animals, such as rabbits and deer, at the bottoms of pipes (so far I haven’t seen any human bones). The animals fell into the holes and could not get out.
The geology is only part of the badlands story. The weather and climate, vegetation, animals, birds, insects, sounds and aromas–all of these, along with the human history and the ranching heritage, work together to complete the story of the badlands.
I think the North Dakota badlands are particularly beautiful because of their parklands; wooded areas that occur in draws and on north-facing slopes. Heavy vegetation in the badlands in places like Little Missouri State Park adds to the scenery. Evergreens, such as the Rocky Mountain juniper, ponderosa, and creeping juniper are interspersed with quaking aspen, cottonwood, and poplar. Limber pines are found in the badlands in the southwest corner of the state, near Marmarth.
I’ve hiked and camped in the badlands many times. Evening summer showers accentuate the colors and the clinker beds assume intense shades of red and orange. The fresh, pungent aroma of wet sage and cedar enhance the experience. At night, the stark, intricately eroded pinnacles can seem unreal. In the moonlight or in a night lightning storm, it is easy to imagine the strange shapes as ruins of a magical city, rather than structures of mere sand and clay. Blend in the sound of coyotes conversing and the badlands environment is complete.
If asked what he or she knows about North Dakota’s geology, an average resident will likely mention the badlands first. That’s true too of visitors, many of whom come to the state to see our best-known natural feature, the scenic badlands along the Little Missouri River.
The badlands landscape is a rugged and hilly one, best viewed from above, looking down on the hills, not up at them, as we usually view buttes. From the rim of the “breaks,” the point where we descend into the badlands, an intricately eroded landscape of sparsely wooded ridges, bluffs, buttes, and pinnacles lies before us. Black veins of lignite coal may be seen eroding out of the steep badlands slopes. Reddish bands of clinker add vivid colors to the area. Pieces of petrified wood, as well as fossil stumps and logs, litter the surface. Behind us stretch rolling plains, interrupted only by occasional buttes.
The American Indians, who inhabited the area when the European settlers arrived, referred to badlands as “mako sica,” (“land bad”). Early French explorers translated and added to this, referring to “les mauvais terrers a’ traverser” (“bad land to travel across”).
General Alfred Sully, preparing to cross the badlands in August of 1864, described them as “hell with the fires burned out.” Theodore Roosevelt, who lived for a while in the Little Missouri Badlands in the 1880s, described them as “fantastically beautiful.” I prefer TR’s description.
Age of the Badlands Materials
Badlands topography is found in several places on the plains of the U.S. and Canada. The best-known badlands in the United States are the extensive “Big Badlands,” along the White River in western South Dakota. Near Dickinson we have the “South Heart Badlands” (known also as the “Little Badlands”) where we find layers of sedimentary rock, equivalent (same materials, same geologic age) in part to those in South Dakota’s Big Badlands. The South Heart Badlands are an erosional remnant of what was once a large butte or group of buttes. The South Heart Badlands are carved mainly from strata of Eocene and Oligocene age, ranging between 55 and 25 million years old. The youngest beds belong to the Miocene Arikaree Formation sandstone (22 million years old), which caps some badlands buttes.
North Dakota’s Little Missouri Badlands extend from near the Little Missouri River’s headwaters in Wyoming near Devils Tower to the point where the Little Missouri River joins the Missouri River in western North Dakota. The materials being eroded in these, our most extensive area of badlands, are much older than those in the South Heart Badlands.
The oldest materials in the badlands are in the southwest corner of the state, near Marmarth, where Cretaceous-age Hell Creek Formation beds (about 65 million years old) have been carved into badlands. The dark and somber, gray and purple beds of the Hell Creek Formation contain dinosaur fossils. Small patches of badlands, carved from the Hell Creek formation can also be seen along State Highway 1806 between Huff and Fort Rice in Morton County.
However, the main area of the Little Missouri Badlands is that which has been carved largely from the Paleocene Bullion Creek and Sentinel Butte formations, which were deposited between 58 and 56 million years ago. The beds that have been eroded into these badlands are too young for dinosaur fossils; the dinosaurs were already extinct when they were deposited.
Between 70 and 40 million years ago, a major mountain-building event known as the Laramide Orogeny (orogeny = “mountain forming”) formed the Rocky Mountains in Montana and Wyoming. As the mountains rose, they were attacked by intense erosion, providing sediment to eastward-flowing rivers and streams. The rivers delivered the eroded sediment to western North Dakota’s coastal plain, an area that could be likened to today’s Mississippi River Delta (central North Dakota was an inland sea at that time). Sediment from the eroding mountains accumulated into thick layers of soft, poorly lithified siltstone, claystone, and sandstone: materials that were deposited on river floodplains and in swamps in what is now western North Dakota. These are the sediments we see exposed today in the Little Missouri Badlands.
In addition to the stream-transported sediments, clouds of volcanic ash, blown eastward from the rising Rocky Mountains during the Laramide Orogeny, collected in layers that were later weathered to clays ( “bentonite”). When wet, the clay absorbs water and swells, and it can become slippery when wet so don’t try walking or driving on it. When the beds dry, they assume a surface texture, similar in appearance and consistency to popcorn, with colors ranging from white to bluish-gray or black.
Why the Badlands Formed
Even though the layers of sedimentary rock exposed in North Dakota’s Little Missouri Badlands range from Cretaceous through Eocene in age (65 to 50 million years old), the badlands themselves–the hills and valleys we see today–are not nearly that old. Before a glacier diverted it, the Little Missouri River flowed northward through a broad, smooth valley, joining the early Yellowstone River in northern Williams County. The Little Missouri and Yellowstone rivers came together near Alamo (about 30 miles north of Williston) in a place now buried beneath 400 feet of glacial deposits. From there, the combined Yellowstone-Little Missouri River flowed northeastward into Canada.
The diversion of the Little Missouri River, away from its route to the north, probably happened sometime prior to the deposition of a volcanic ash bed on the glacial sediment blocking the channel (the ash was deposited as a result of a volcanic eruption in the area of Yellowstone Park 640,000 years ago). It is possible, though, that an earlier glacier might have diverted the river – the 640,000-year figure is a minimum date; erosion of the badlands may have begun as early as 3.5 million years ago.
Since it was diverted by glacial ice, the Little Missouri River has flowed over a shorter and steeper route than it did prior to its diversion. That part of the river’s route today, from the point where it makes its sharp turn toward the east in the area of the North Unit of Theodore Roosevelt National Park, is east rather than north as it had been before a glacier diverted it. When the river assumed its new, shorter route toward the Gulf of Mexico, it began a vigorous erosion cycle, cutting down more rapidly and deeply and sculpting badlands topography. The badlands then, are an indirect result of glacial activity, even though the only conspicuous direct evidence of glaciation remaining in the area is an occasional glacial erratic on the upland in northern McKenzie County.
Late in the Cretaceous, beginning about 70 million years ago, and continuing through the Paleocene, until about 56 million years ago, western North Dakota’s climate was subtropical. Trees up to 12 feet in diameter and more than 100 feet tall grew in a setting similar to today’s Dismal Swamp in Virginia, or the Florida Everglades, with meandering rivers, swamps, and vast forested floodplains. Modern evidence for this fossil forest includes widespread seams of lignite coal, fossil tree leaves, pollen, and logs and stumps of petrified wood.
Lignite is a soft coal that underlies much of the western two-thirds of North Dakota. It began as an accumulation of dead plant material in tropical or semitropical basins: swamps, lagoons and marshes. As the basins filled with stagnant water, the plant debris became submerged so that atmospheric oxygen could not reach it. When the plants died and fell into the water, they began to decay, but before all the plant debris could decompose, the bacterial action causing the decay stopped; most of the bacteria “committed suicide” by filling the stagnant swamp water with their own toxins to such an extent that they died. The only bacteria that remained were ones that did not need oxygen for respiration. However, these “anaerobic” (the word means “living without air”) bacteria are less efficient at decomposition. As a result, large amounts of submerged organic materials did not decompose, and thick beds of peat accumulated.
Streams meandering through western North Dakota during Paleocene time changed course frequently and, when they did so, they sometimes deposited sand and silt on top of the partially decomposed vegetation (peat). The layers of peat were buried beneath thick layers of sediment and the weight of the overlying beds gradually compressed the peat to lignite. Layers of swamp vegetation, some of them over 50 feet thick, were eventually transformed into beds of lignite coal only a few feet thick.
Seams of lignite, horizontal black bands, can be seen eroding out of hillsides today. They range from a few feet to as much as forty feet thick in Slope County and even thicker in Wyoming and Montana. If a peat bog happened to be buried by river sediments before the decay process had progressed very far, and trees were still growing in the swamps, some lignite may have formed, but some of the trees were instead changed into petrified wood. Occasionally, a petrified tree stump, rooted in a lignite bed, can be seen.
Petrified wood formed when minerals gradually replaced the buried plant material. The petrification process requires rapid burial of the wood to prevent decay. This sometimes happened when rivers shifted course or overflowed their banks, burying a forest floor under a layer of sand and silt. Other times, forests were partially covered by volcanic ash, blown to the area from volcanoes in the rising Rocky Mountains. After burial, ground water seeped through the ash and wood, coating cell walls and filling the intercellular cavities with minerals.
Usually, the cellular structure of the wood was destroyed; leaving only a rough cast of the original log, but sometimes growth rings, bark, knots, and even the shapes of the wood’s tiny cells are preserved with remarkable fidelity. This more detailed preservation is possible because some molecules, such as silica and other inorganic materials, are much smaller than organic molecules so, rather than “molecule for molecule” replacement, the organic molecules are coated and surrounded with silica. Cavities in petrified wood may be encrusted with quartz crystals.
Petrified wood ranges from solid, well-silicified specimens to splintery, or “coalified” wood that tends to disintegrate when it is exposed to weathering or it may simply fall apart when you pick it up. The degree of petrification can vary, even within a single specimen. Individual stumps or logs may contain both well-silicified parts and other parts that are still coal. Most of North Dakota’s petrified wood is brown or tan on weathered surfaces and dark brown where freshly broken, but colors can range from white to gray, with streaks of black. Traces of minerals add color to the fossilized wood: yellow, brown and red may indicate iron; black and purple hues suggest carbon or manganese mineralization.
Petrified wood occurs as entire logs or stumps, some standing upright where they once grew, or as scattered limbs and fragments, strewn over the land surface. A fallen log was probably cylindrical when it fell down, but the petrified logs we find today often have oval cross sections because, after they were buried, they became compressed and flattened by the weight of overlying sediments. Most of North Dakota’s fossil wood is Paleocene in age, but petrified wood is also found in smaller amounts in the older Hell Creek Formation and in some of the younger bedrock units.
Fossil leaves, commonly found along with petrified wood, help us to identify the species of trees that grew in and near the swamps where petrified wood is found. Many specimens belong to the plant genus Metasequoia, the dawn redwood. Fossils of dawn redwood were first discovered in 1941, and the tree was thought to be extinct, but living specimens were discovered in south-central China in 1945. Today, the dawn redwood is widely used as an ornamental tree in warmer climates.
During the Paleocene, while Metasequoia trees were growing in North Dakota, a variety of other kinds of vegetation were also present. We know them primarily through studies of fossil pollen and the delicate imprints of leaves in mudstone, siltstone, and carbonaceous shale. Along with the leaf fossils, we find remarkably preserved petrified cones of Sequoia dakotensis (giant evergreen trees), the leaves of tree ferns, and various kinds of petrified wood.
So much fossil wood is strewn over the surface in some places that such areas are referred to as “petrified forests.” North Dakota’s best-known petrified forest is in the South Unit of Theodore Roosevelt National Park, where large numbers of tree stumps have eroded out of the Sentinel Butte Formation. Some stumps are still upright, in the positions in which they grew 60 million years ago. They were preserved when the forest floor was flooded, burying the bases of the trees. The unburied parts of the trunks and branches decayed and disappeared. Petrified stumps may be anchored in a lignite bed or a buried soil horizon, which may mark a former forest or swamp floor.
Petrified wood is often used in landscaping. Many western North Dakota driveways and flower beds are decorated with fine specimens. An outstanding example of a petrified stump, collected in McKenzie County, may be seen in the Long-X Visitor Center in Watford City. The stump, probably bald cypress, is nine feet in diameter and weighs about eight tons. Perhaps the most elaborate use of petrified wood in an ornamental sense is in the Petrified Wood Park in Lemmon, South Dakota. In this park, completed in 1932, O. S. Quammen constructed hundreds of pillars and intricate structures of petrified wood, much of it from North Dakota.
In 1990, the level of Lake Sakakawea was low, revealing several petrified logs weathering out of the Sentinel Butte Formation along the lake shore in Mercer County. Pieces of an 80-foot-long petrified log, collected from the area, along with two stumps from the Amidon area, are displayed on the North Dakota State Capitol grounds. The log and stumps were located southeast of the State Capitol building, in the Centennial Grove for many years, but they were moved to a location east of the Heritage Center in 2014. Still another large petrified log was uncovered during construction of Interstate Highway 94 west of Dickinson, This 120-foot-long, six-foot diameter log (much larger than the one on the State Capitol grounds) was offered to nearby towns as a tourist attraction, but it was reburied when no one wanted it.
Glaciation was the main geologic influence on much of North Dakota’s landscape. The Ice Age, a time geologists also refer to as the Pleistocene Epoch, includes most of the past three million years of geologic time. Glaciers advanced over the northern plains several times during the Ice Age, reaching northern and eastern North Dakota. When it wasn’t glaciated, the state had a climate much like the one we enjoy today or possibly even milder at times. the Ice Age wasn’t one long “deep-freeze.”
During their studies of the geology of the state, geologists have found evidence for at least seven separate glaciations, but there may have been more. The most recent of these glaciations is known as the Wisconsinan (because deposits typical of that glaciation are widespread in Wisconsin). The Wisconsinan glaciation began about 100,000 years ago and ended about 11,000 years ago. Some geologists debate whether the Ice Age has really ended yet. After all, large areas of the earth’s surface are still covered by extensive glaciers (Greenland, Antarctica, etc.). It’s likely that we are currently enjoying a lull between major glaciations.
Even though North Dakota was glaciated many times during the Ice Age, it is the Wisconsinan glacial deposits, the most recent ones, that are most obvious to us. These are the ones that form the hills and valleys in eastern and northern North Dakota and they are the ones in which our prairie potholes and wetlands are developed. Most of our richest farmland is developed on the Wisconsinan glacial surface.
Early glaciers, which advanced into North Dakota before the Wisconsinan glacier, also had a profound effect on the state. The materials they deposited have been largely eroded away, and about all that remains of them are occasional boulders — “erratics.” I will discuss erratics elsewhere. It was an early glacier that diverted the course of the Little Missouri River eastward more than 640,000 years ago (possibly earlier). Before that time, the Little Missouri River flowed northward into Canada. In fact, all of North Dakota used to be drained by rivers that flowed into Canada. When it was diverted, the Little Missouri River began to carve the badlands we see today.
All of us who have traveled around North Dakota know that the landscape varies considerably from place to place. Southwestern North Dakota, with its badlands, buttes, and broad vistas is largely the result of hundreds of thousands of years of erosion. The landscape there is not glacial. It has been carved from layers of flat-lying sandstone and other materials.
The Missouri River marks an approximate boundary between the eroded landscape of southwestern North Dakota and the entirely different glacial landscape north and east of the river, where we see small hills – small at least compared to large buttes like Sentinel Butte and Bullion Butte found in southwestern North Dakota. Eastern North Dakota is characterized by thousands of potholes, poorly developed drainage in places, and remarkably fertile farmland.
When the glaciers advanced over the state, they picked up some of the materials over which they flowed. The glaciers contained a variety of kinds of soil and rock, which they eventually deposited as thick layers of sediment. The exposed surface of these sediments has been weathered for the past several thousand years (since the glaciers melted) and it forms the rich soils our farmers work today.
Over much of eastern North Dakota, the glacial sediments were laid down as an undulating plain (think of the Carrington, Finley or Kenmare areas, for example). In other places, a more hilly landscape resulted (think of Turtle Mountain or the Missouri Coteau — places like Belcourt, Hurdsfield, Max, Ryder and countless others). In still other places, water from the melting glacier became ponded, forming huge lakes. Today, most of these areas are flat. Examples of the flat topography may be seen in places like Fargo, Hillsboro, Grand Forks or Grafton. The old floor of Glacial Lake Agassiz (the Red River Valley) is the classic example of flat. Hundreds of smaller glacial lake plains are found in North Dakota too.
As the glaciers flowed over North Dakota, they tended to smooth off and wear down the hill tops and fill in the lower areas with sediment. The overall result is a fairly level landscape. The layers of glacial sediment underlying that landscape are extremely complex, containing buried river channels, blocks of sandstone and shale, old landscapes that were covered many times by fresh glacial sediments. Buried layers of gravel and sand, deposited by water flowing from the melting glacial ice, constitute aquifers. They contain some of our best sources of fresh water.
As the ice flowed, in some places it picked up large chunks of material and moved them short distances before setting them down again. A good example of this is at Devils Lake, where a large amount of material was picked up and moved southward a few miles. Today, Devils Lake lies in a broad lowland. South of the lake is a high range of hills, including Sully’s Hill. The hills consist of materials that were once in the lowland where Devils Lake is now.
In some places, huge floods of water from melting glaciers carved deep river channels. Countless small meltwater valleys, along with some large ones too, are found throughout eastern and northern North Dakota. The Sheyenne, Souris, and James River valleys are good examples of large meltwater valleys. Valley City, Minot, and Jamestown are nestled in meltwater valleys. The Missouri River valley is another example of a glacial river channel, but it had such a complicated history that I’ll plan on writing a special article about it.
How thick were the glaciers that covered North Dakota? Certainly, they were more than a thousand feet thick in the east and north, so thick that the Earth’s crust beneath the ice buckled and sagged downward, eventually rebounding when the ice melted.
Who or what lived in North Dakota during the Ice Age? Mastodons and wooly mammoths lived along the edge of the glacier. Elk, caribou, and horses were common. Horses became extinct in North Dakota and in North America at the end of the Ice Age, They survived, worldwide, because they had migrated to Asia via the land bridge between North America and Asia prior to then. During my field work over the years, I’ve found mastodon teeth, caribou bones and, in the Lake Agassiz deposits, fossil fish bones, mainly perch. It’s likely that early humans also lived here while the most recent glacier was still melting.
I’ve mainly been discussing North Dakota’s glacial landscape. Part of the state, the southwest quarter, was not glaciated, but the glaciers also left their mark there. The badlands along the Little Missouri River owe their existence to early glaciers that diverted the river eastward from its northerly route into Canada. This diversion triggered greatly increased erosion by the Little Missouri River, which resulted in the formation of badlands. Some places that were not glaciated are marked by polygons, formed when permafrost froze the land beyond the limit of the glaciers.