The work of John Bluemle PhD

Burleigh

13-EARTHQUAKES

Governor William L. Guy’s secretary thought she felt a sonic boom on Monday, July 8, 1968. The State Capitol Building shook a bit, but most people did not feel the shaking or, if they did, they did not recognize it for what it was, a 4.4-magnitude earthquake. The earthquake was centered just southwest of Huff and it was felt over a 3,000-square-mile area. In Huff, and on nearby farms, the quake rattled dishes, window and wood-frame houses creaked, but no damage was reported anywhere in the state.

At least 12 additional earthquakes have been felt in North Dakota. The most widely felt earthquake in North Dakota occurred at about 9 p.m on May 15, 1909. It was a shock that rocked the northern Great Plains. The epicenter of this tremor was near Avonlea, Saskatchewan, near the North Dakota-Montana- Saskatchewan border. The Avonelea earthquake was felt throughout North Dakota and western Montana as well as in the adjacent Canadian Provinces. It broke windows and dishes and cracked plaster and masonry.

Some of the largest earthquakes in U.S. history, in the early part of the 19th century, were likely felt in North Dakota. They are known as the New Madrid quakes, after the town of New Madrid in southeastern-most Missouri. A series of four strong quakes (and hundreds of smaller ones) occurred – two on December 16, 1811, and one each on January 23, 1812 and February 7, 1812. The largest of the quakes was felt from the Gulf of Mexico to Canada and from the Rocky Mountains to the Atlantic coast. The potential remains for more devastating earthquakes in the New Madrid area, and if that happens, we will likely feel it in North Dakota.

Other earthquakes that have been centered and felt in North Dakota include one in the southeastern part of the state in 1872; one near Pembina in 1900; three in the Williston area in 1915, 1946, and 1982; and one each in the Hebron area in 1927; near Havana in 1934; and the Selfridge area in 1947. Earthquakes centered near Morris, Minnesota were felt in southeastern North Dakota in 1975 and 1993.

Almost all earthquakes are caused by sudden slippage along faults in the upper few hundred miles of the Earth’s outermost shell (the crust). Most of them occur at the boundaries between the several large plates, which fit together to form the crust. These plates move, in some places pulling away from one another, as along the spreading parts of the mid-Atlantic Ridge, sliding past one another, such as along at the San Andreas Fault in California, or colliding into one another, such as in the Pacific Northwest (the Cascadia Fault in British Columbia, Oregon, and Washington) where the Pacific Plate is pushing under the North American Plate. In all of these, and other comparable areas, the potential is higher for a severe earthquake.

normal fault, earthquages, geology, North Dakota

Fig. 13-A. “Normal fault” is one in which one block moves down relative to the other. This happens if the two blocks, on either side of the fault plane, are being pulled apart. Examples of such normal faults in the U.S. are the Basin and Range (including the Wasatch fault in Utah-which has had historic earthquakes), the Napa Valley Fault in California, and the New Madrid area along the Mississippi River (a great earthquake in early 1800s).

Movement along these plates is slow but steady, most of the time approximating one to ten millimeters per year (about the rate at which your fingernails grow), but the continual slow movement causes stress to build. When the stress finally exceeds the strength of the rocks, they break and snap violently into a new position (the last time that happened in the Pacific Northwest was in 1700).The point of rupture, which may be many miles beneath the Earth’s surface, is known as the focus or hypocenter of an earthquake; the epicenter is the point directly above the hypocenter. The process of breaking (known as faulting) creates vibrations called seismic waves. We feel these waves as earthquakes. Earthquakes can occur anywhere enough elastic strain  builds up to drive fracture propagation along a fault plain.

The sides of a fault may move past each other smoothly, without causing an earthquake if there are no irregularities along the fault surfaces. However, nearly all faults do have irregularities, which tend to cause frictional resistance. If a fault becomes locked–“stuck” in place, continued relative motion between the plates leads to increasing stress, and therefore, stored strain along the fault surface. This continues until the stress has risen sufficiently to break through the friction, allowing sudden sliding over the locked portion of the fault, releasing the stored energy.

Large earthquakes are among the most devastating natural events that can occur. If an earthquake occurs near the edge of a continent, it may generate a tsunami, which can result in a massive flood when it comes ashore. Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean, the distance between the wave crests can be more than 60 miles, and the wave periods can vary from five minutes to an hour. A tsunami wave can travel from 375 to 500 miles per hour, faster in deep water, slower if the water is shallow. Tsunamis can travel thousands of miles across open ocean, inundating far shores several hours after the actual earthquakes that generated them. One of the most devastating tsunamis in recent years occurred on December 26, 2004 as a result of a massive earthquake (known as the Sumatra-Andaman Earthquake), with a Richter Scale rating of 9.3 off the west coast of Sumatra, Indonesia. The earthquake that triggered the tsunami was the third largest earthquake ever recorded and it also had an unusually long duration of shaking, about ten minutes. The resulting 100-foot-high tsunami wave killed more than 230,000 people when it came ashore in Sumatra and Thailand.

Other recent, large earthquakes include one in 1960 in Chile and one in 1964 in Alaska (the Good Friday Earthquake). The most devastating (in terms of casualties) earthquakes in recorded history were the 1556 Shaanxi earthquake in China, which killed 830,000 people, and the 1976 Tangshan earthquake, also in China, which killed 655,000 people. The fatalities in the Chinese earthquakes were due to direct earthquake damage, not tsunamis, which more often account for most of the fatalities.

Seismic waves (from the Greek seismos, meaning “caused by an earthquake”), actually consist of two kinds of waves: surface waves and body waves. Body waves travel deep into the Earth’s mantle, and even through its core, before reaching the surface, whereas surface waves travel along the Earth’s surface.

fault, earthquake, geology

Fig. 13-B. A “thrust fault” is one in which the two blocks are being pushed together. One will push up, over the top of the other – or one might be pushed beneath the other. An example is the Cascadia Fault along the northwest US-Canada coast, where a plate is pushing inland (Oregon, Washington, British Columbia). In Asia, the India sub-continent is pushing northward, forcing the crust ahead of it to buckle, forming the Himalayan Mountain Range.

The seismic waves of large earthquakes can induce natural oscillations in the Earth and cause the entire planet to ring like a bell for hours, or even days. The reverberating tone is much too low for us to hear, but seismographs can record the low-frequency oscillations. The recorded sound can be played back at, say, 10,000 times faster than the original. We can then, as it were, listen to the Earth. It is a strange experience that sounds like being in a forest on a windy day, with occasional brief falling tones and longer, rather melodic tones, similar to an orchestra tuning up. Every now and again we hear sharp noises that sound like a branch breaking. Sometimes we hear sounds like a herd of animals stampeding through a forest, smashing off branches and breaking them underfoot. I have listened to several such recordings. You can access samples of earthquake sounds on a variety of internet sites.

Most earthquakes that originate in North Dakota are probably related to deeply buried structures in the Precambrian basement. These structures contain numerous faults, but because they are so deeply buried, their extent and locations are poorly known. Movement on any of the faults could produce small to moderate earthquakes. Small earthquakes can also occur when layers of sedimentary rock collapse into voids left by the dissolution of underlying salt beds. Northwestern North Dakota is underlain by thick layers of salt at depths ranging from 4,000 to 12,000 feet. Salt is a geologically unstable mineral, readily dissolved in water and, when burdened under the tremendous mass of overlying sediments, it can flow and deform. As the salt moves, the support for overlying layers may be removed. The overlying layers can settle downward gradually, or they may collapse suddenly, creating a comparatively shallow, small earthquake.

Seismographs around the world record earthquakes with a magnitude of about 4.5 or greater; seismic waves of smaller tremors dissipate before being recorded by distant instruments. For an earthquake with an epicenter in North Dakota to be recorded, it would have to have a magnitude of 3.3 or greater. The 1968 Huff earthquake, which I mentioned earlier, is one of only about a half-dozen that have been instrumentally verified to have epicenters in North Dakota, although it is likely that other small reported earthquakes have had epicenters within the state. Tremors of Richter magnitude 3.0 or less are often felt by persons favorably situated, so more small tremors could have occurred in the state than instrumentally-verified records suggest. The only permanent seismic monitoring station in North Dakota is located near Maddock, southwest of Devils Lake.

In 1978, along with Alan Kehew, Erling Brostuen, and Ken Harris, I investigated reports by farmers of loud, “banging” sounds in Emmons and Dickey counties in south-central North Dakota. It was a drought year and cracks formed in fields of alfalfa. We determined that the alfalfa, which has a particularly deep root system, was de-watering the soil, causing it to shrink. The cracks – and the noises – were due to shrinkage of the soil, which caused deep cracks to form. As the cracks formed, material at their edges fell into them, many of which were 10 to 15 feet deep. The banging noises may have been due either to material falling into the cracks or, perhaps, the soil may have pulled apart with force as it shrank, causing the sounds. The occurrences I just described are not earthquakes, as they do not involve tectonic forces. They might be characterized as examples of “pseudo-earthquakes,” or perhaps “soilquakes.” Similar events – landslides, etc. – might also be mistaken for earthquakes. We published the results of our “alfalfa study” in a scientific journal: (Bluemle, J. P., Kehew, A. E., Brostuen, E. A.,. and Harris, K. L., 1978, Alfalfa and the occurrence of fissures on the North Dakota prairies, The Prairie Naturalist, Vol. 10, pages 53 – 59).

As a sort of afterthought, because I am so often asked about it, I’ll add a note on current concerns about possible earthquakes being triggered by hydraulic fracturing (“fracking”) activity in our oil-producing areas. At the depths at which they are performed in North Dakota, hydraulic fracturing procedures are unlikely to cause earthquakes. It is possible, though, that injection of waste fluids into certain geologic formations could trigger small earthquakes, as has been reported in some places (Texas, Oklahoma, Ohio, etc.). The likelihood of a damaging earthquake due to this activity in North Dakota is remote, although pollution of groundwater is possible.

strike-slip fault, earthquake, geology, North Dakota

Fig. 13-C. “Strike-slip fault” might result in an earthquake if the right-hand block moves parallel to the bedding, but not up or down relative to the material on the other side of the fault. This is the main kind of movement that occurs along the San Andreas Fault System along the west coast of the United States.

The closest I ever came to directly experiencing a significant earthquake was on September 26, 1997, when my wife, Mary, and I were in Assisi, Italy. We spent several hours sightseeing in Assisi, some of it in the Basilica of St. Francis of Assisi. That evening, while we were staying in Chianciano Terme, a nearby village, an earthquake caused extensive damage in Assisi, killing several people in the church we had been in a few hours earlier.

The United States Geological Survey lists North Dakota among ten states that are least likely to suffer earthquake damage. Some other nearby “earthquake-poor” states include Iowa, Minnesota and Wisconsin. Infrequent, small earthquakes may occur near to and within North Dakota, but unless one occurs in a remarkably unfortunate location, it is unlikely that any serious damage will occur. It ought to be safe to visit churches here.

 

4-LIGNITE AND PETRIFIED WOOD

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.

Petrified stump

Fig. 4-A. Petrified stump. South Unit, Theodore Roosevelt National Park, Billings County. The badlands topography in the background is carved from the Bullion Creek Formation. Photo: 9-09-2009.

Lignite exposure

Fig. 4-B. Lignite exposure about six miles southeast of Medora, Billings County. The lignite occurs at the top of the Bullion Creek Formation, which appears light gray in the exposure. The Sentinel Butte Formation, more brown in color, overlies the lignite. Photo 9-09-2009

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.

Lignified tree trunk

Fig. 4-C. Tree trunk rooted in lignite beds. The tree is buried by silt and sand, which were deposited quickly, thus preserving the tree trunk. The vertical part of the tree grades upward, from lignite at the base, to a combination of lignite and some petrified wood. The pick ax is about four feet long. Photo: 1979

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.

North Dakota, Park, petrified stump

Fig. 4-D. In 1973, along the South Loop Road for Theodore Roosevelt l Park. It was later destroyed by vandals.

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.

Elgin ND Petrified Wood Bandstand

Fig. 4-E. Elgin ND Park bandstand, Grant County. The wall is constructed of petrified wood collected in the area. This is a good place to see a variety of nice specimens. Photo: 6-23-2010

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.

Petrified Wood Park, Lemmon, South Dakota

Fig. 4-F. Petrified Wood Park, Lemmon, South Dakota (about a half mile south of the ND-SD state line). Constructed by Ole S. Quammen, the park covers a city block, and was completed in 1932. The petrified wood was collected in both North and South Dakota. Photo: 6-24-2010

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.

Petrified log

Fig. 4-G. Petrified log displayed on the Capitol campus, Bismarck, prior to being relocated. This 80-foot-long log was collected along the shore of Lake Sakakawea in Mercer County when the reservoir level was low. Even though the tree trunk was round when it fell, the cross-sectional shape now is oval rather than round. The weight of the overlying sediment in which it was buried caused the log to assume its oval shape. Photo: 9-26-2009.

 

3-GLACIAL ERRATICS: NORTH DAKOTA’S WANDERING ROCKS

Fountain at Crystal Springs

Fig. 3-A. Fountain at Crystal Springs, Kidder County. This field-stone monument was constructed in 1935 by stonemason Art Geisler. It was a Works Progress Administration project intended to replace an old iron pipe from which travelers used to obtain a cool drink of spring water while motoring on U.S. Highway 10. The fountain was listed on the National Register of Historic Places in 2010. The spring was flowing when I took this picture. The water flows from an ice-thrust hole, which intersected a preglacial river valley. Photo: 8-30-2010.

Field stones are common in parts of North Dakota that have been glaciated. Early settlers used the stones for the foundations of their homes and farm buildings and some people built entire structures with them. Today, field stones are used in landscaping, as rip rap along the faces of dams and shorelines, or as decorations in front yards in towns like Bismarck and Minot (less so in places like Fargo and Grand Forks, where they are much less common).

North Dakota, geology, erratics, Forbes, Schulstad House

Fig. 3-B. House built of field stones in Forbes, in Dickey County on the ND-SD state line. An inscription reads “Schulstad House: 1907.” Photo: 7-8-2010.

 

 

 

 

 

 

 

Geologists use the term “erratic” to refer to field stones left behind by glacial ice. The term “erratic,” with reference to rocks, dates to 1779, when Horace de Saussure, a Swiss geologist, described granite boulders lying on top of limestone in the Jura Mountains in Switzerland. He recognized that the boulders were out of place. His term, “terrain erratique,” comes from the Latin erratus, “to wander,” and means, literally, “ground that has wandered.”

In some instances, the source-area of an erratic can be pinpointed. North of Winnipeg, for example, several Paleozoic carbonate limestone formations are quarried. We can determine from which area and formation a North Dakota boulder was derived by matching it to the Manitoba limestone exposures.  Several years ago, Bob Biek, then a North Dakota Geological Survey geologist, found a number of unusual erratics along Lake Sakakawea — dark-colored stones known as “omars.” The name “”omar” is short for the Omarolluk Formation, a 1.76 billion-year-old greywacke formation. The rocks are found in-place (where they originally formed) today only in the Belcher Islands in southeastern Hudson Bay, so it is possible that the Lake Sakakawea omars originated in the Belcher Islands, or near there. The Belcher Islands are located nearly 1,000 miles northeast of Lake Sakakawea.

granite erratic, field stone

Fig. 3-C. Large granitic erratic about ten miles west of Mandan, Morton County. This is one of the largest erratics in North Dakota. Several pieces of granite, some of them rising ten feet above the ground surface, are all part of what was once a single erratic, which weighed at least 350 tons (but I don’t know how much additional rock may be buried beneath the ground surface). Numerous large granite erratics occur in this part of eastern Morton County. They are all that remains of materials deposited during an old (pre-Wisconsinan) glacial advance.
Old Highway 10 can be seen on the right. My wife, Mary (5’7”) shows the scale. Photo: 6-5-09

Erratics have been used as exploration tools in the search for ore deposits. Copper mines were opened in Finland after copper-bearing erratics were traced back to their source. Analysis of gold-bearing erratics in Maine resulted in the discovery of gold ore deposits in Quebec. I have found occasional erratic boulders in North Dakota containing traces of gold. Such erratics were probably transported to North Dakota from the metal mining districts of Manitoba and Saskatchewan, about 700 miles to the north.

riprap, Devils Lake

Fig. 3-D. Riprap of erratics along a Devils Lake levee built to protect the town of Minnewaukan, Benson County. Photo: 9-13-2011

 

 

 

 

 

 

 

 

 

Glacial erratics represent the oldest geologic materials found on the surface in North Dakota. Those composed of limestone or dolomite are mainly from 300 to 500 million years old, while some of the igneous or metamorphic erratics may be three or four billion years old. In contrast, the land surface they are lying on could be as young as 12,000 years old in places where erratics lie directly on glacial deposits.

In contrast to the long-distance travelers, boulders of sandstone were moved no more than a few miles by a glacier from nearby locations within the State. Sandstone is less well consolidated than granite or limestone and any extensive glacial transport of sandstone boulders would break them down into smaller fragments, or reduce them to sand. Occasionally, boulders of shale are included in layers of glacial sediment. Most such boulders are quite fragile and have probably been moved only a few tens or hundreds of feet from their original source.

erratics, boulders

Fig. 3-E. Field with abundant erratics, about six miles east of Wing, northeastern Burleigh County. Photo: 9-4-10.

The larger erratics, those three feet or more in diameter, tend to be igneous or metamorphic rocks, such as granite or gneiss. Such rocks are hard and much more resistant to abrasion and fracturing than are sedimentary rocks such as limestone. In some places, especially large granite or quartzite erratics, ten feet or more in diameter are numerous (some are car-sized, measuring up to 20 feet across). A few examples include the walls of the Sheyenne River Valley near Fort Ransom; many of the high bluffs along the Missouri River; along the White Earth River Valley in Mountrail County; and in the valley walls along the Souris River in and near Minot and Velva. Both large and small erratics are particularly abundant near Venturia and Zeeland in McIntosh County. The largest erratic I have seen is located eleven miles south of Calgary, Alberta. Composed of quartzite, and known as the Okotoks Erratic (aka “Big Rock”), it weighs 16,500 tons, stands 30 feet above the surrounding area and is billed as the world’s largest glacial erratic.

Erratics tend to be abundant in places where the ground surface has been washed by the winnowing action of waves along the shores of glacial lakes and modern reservoirs. Wave action removes the finer materials, leaving a lag of cobbles and boulders behind. Examples include areas along the wave-worn shore of glacial Lake Agassiz, near Pisek in Walsh County and Hankinson in Richland County. Erratics are sometimes concentrated along the shores of modern lakes, such as Lakes Addie and Sibley, near Binford in Griggs County and along Devils Lake in Benson County (but many of the erratics along Devils Lake are now submerged). A good place to see erratics is along the levees and causeway roads that have been constructed in response to Devils Lake flooding. Great numbers of erratics have been brought to the area to serve as rip rap along shorelines subject to wave erosion.

boulder pavement

Fig. 3-F. Road cut along State Highway 32 north of Niagara, Grand Forks County, exposing a boulder pavement lying on Cretaceous Pierre Formation shale (dark gray beneath the stones) and overlain by till deposits. Scanned photo: 1968

Most erratics are rounded and worn, but some of them have beveled or faceted surfaces. During the course of their journey, the rocks were jostled against one another while in the glacial ice, or against the rock over which the glacier was flowing. As a result of this rubbing, the surfaces were planed smooth. Glacial transport fractured some boulders, producing fresh, angular edges. Some erratics are grooved or polished, a result of abrasion by the moving ice. Coarse sand and gravel within the ice scraped against the boulders, scratching or “striating” them, sometimes as the boulder moved along with the advancing glacial ice or when the glacier flowed over a hard, stationary rock.

In some places where the more-easily eroded glacial deposits have been largely eroded away, erratics may be concentrated on the land surface (eastern Burleigh and western Kidder counties are examples), resulting in a very bouldery landscape. If such a landscape was then glaciated again, and covered by fresh glacial deposits (Late Wisconsinan glacial deposits lying over Early Wisconsinan glacial deposits, for example), the erratics may occur as a buried boulder zone, known as a “boulder pavement.” Boulder pavements are common, but not often discovered, unless an excavation cuts though the boulder zone. This is most likely to happen during road construction. Striated boulders with straight grooves are sometimes found in such “boulder pavements.” If the boulders have not moved, the striations can sometimes be used to determine the direction of glacial flow.

buffalo boulder

Fig. 3-G. “Buffalo boulder” rubbing stone, about five miles southeast of Napoleon, Logan County. This boulder is about four feet in diameter. Photo: 5-30-2010

Single large, isolated erratics are sometimes surrounded by depressions, a result of animals such as bison or cattle using them as rubbing stones. Such “buffalo boulders” form as animals rub against the stone, loosening the soil with their hooves. The wind blows the loose soil away, leaving a depression surrounding the rock. Many buffalo boulders are polished from repeated rubbing by the animals.

Erratics aren’t restricted to the surface. They occur throughout the entire thickness of glacial sediments, which averages between 150 and 250 feet thick throughout the northern and eastern parts of North Dakota. Seasonal freezing and thawing causes rocks to work their way upward to the surface from below the plow zone. Every farmer knows that, each spring, a new “crop” of stones has to be removed from the fields. The smaller rocks can be picked up with rock-picking equipment and carried away. Larger erratics are sometimes blasted with explosives and the pieces hauled away. Some of the very largest are simply left in place and avoided.

Standing Rock monument

Fig. 3-H. Standing Rock monument along State Highway 46 in Ransom County. The monument is placed on top of Standing Rock Hill, a prominent ice-thrust hill. Photo: 7-17-2009.

Some erratics are famous. Everyone has heard of Plymouth Rock where the Pilgrims first set foot in the New World on December 21, 1620. In North Dakota we have the Standing Rock, where Highway 46 crosses the Sheyenne River Valley near Fort Ransom. The explorers Nicollet and Fremont, in 1839, noted Standing Rock Hill  on their maps. In northwestern North Dakota, near Alkabo in Divide County, is Writing Rock, which was known by the Sioux as Hoi-waukon or Spirit Rock.

In his book, Blue Highways, a Backroads Tour of Rural America, William Least Heat Moon captured the resignation of farmers to a continual crop of boulders:

East of Fortuna, North Dakota, just eight miles south of Saskatchewan, the high moraine wheat fields took up the whole landscape. There was nothing else, except piles of stones like Viking burial mounds at the verges of tracts and big rock pickers running steely fingers through the glacial soil to glean stone that freezes had heaved to the surface; behind the machines, the fields looked vacuumed. At a filling station, a man who long had farmed the moraine said the great ice sheets had gone away only to get more rock. “They’ll be back. They always come back. What’s to stop them?”

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