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.
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.