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.
Eskers and kames are among the best-known of the various features formed by glaciers and by the running water associated with melting glaciers. Eskers come in all sizes: ridges snaking across the countryside ranging from a few hundred feet to several miles long, and up to 50 or 100 feet high. Kames may be cone or pyramidal-shaped hills as high as a hundred feet, or they may be simply small mounds of material. Kames and eskers are found in most parts of North Dakota that were covered by the Late Wisconsinan glacier.
Eskers were deposited by streams and rivers flowing 1) on the surface of a glacier, 2) in cracks in the glacial ice or, sometimes, 3) in tunnels beneath the ice. Imagine a river flowing in a valley or crack in the glacier. The banks of the river were formed of ice and, in some places, the river floor might also have been on ice. These Ice-Age rivers and streams deposited gravel and sand in their ice-walled valleys, just as a modern stream deposits sediment in its earthen valley. However, the ice banks of esker rivers eventually melted away, leaving the gravel deposits that had been deposited in the ice-walled valleys, standing as ridges above the surrounding countryside.
North Dakota has thousands of esker ridges. Most are small and non-descript, apparent only from a height (air view) or on air photos, but some eskers are impressive.
One of the best examples of an esker in the U.S. is the Dahlen Esker, located midway between Fordville and Dahlen in the northeastern part of the state. It can be seen as a prominent ridge off to the west where State Highway 32 crosses the Grand Forks-Walsh County line. If the weather is dry, you can drive about a mile on a section-line trail to the crest of the Dahlen Esker (if the fields on either side of the trail are being farmed, be careful not to drive on the crops).
The Dahlen Esker was deposited by a meltwater stream flowing in an ice-walled channel, or possibly through a tunnel in the ice, near the edge of the glacier. The stream flowed mainly southward, toward the margin of the glacier. The esker is about four miles long, 400 feet wide, and as high as 50 to 80 feet. In some places, native prairie covers the surface. The Dahlen Esker has been described in various geologic reports dating to the late 19th century, but the best description and discussion of the feature was provided by Jack Kume for the North Dakota Geological Survey (“The Dahlen Esker of Grand Forks and Walsh Counties, North Dakota,” Miscellaneous Series 32, 1966).
Other large eskers include ones near Benedict in northeastern McLean County; another about ten miles southwest of Carrington in Foster County; one immediately west of Hannaford in Griggs and Barnes counties; one near Dazey in Barnes and Stutsman counties; and an unusual one in Frankhauser Lake in northern Sheridan County. The Frankhauser esker is particularly interesting because it winds its way through a lake, which floods a depression that was formed by ice-thrusting. The esker formed during the ice-thrusting process.
Kames are similar in many ways to eskers. Like eskers, they consist largely of gravel and sand, but they are conical or irregularly shaped hills, rather than long ridges.
Water flowing on the surface of the glacier, or in esker valleys, plunged into holes in the ice, filling the holes with a mixture of materials. When the ice eventually melted, the water-deposited materials slumped down, resulting in mounds and conical hills. Kames occur in several places, mainly on the Glaciated Plains, usually in association with ground moraine, but they are sometimes found in areas of dead-ice moraine. A few examples include one in southwestern Richland County, one south of Lidgerwood, and one west of Cayuga in Sargent County. A prominent kame, visible from miles away, is located four miles south of State Highway 200 in eastern Foster County along the west side of the James River. About half of the 60-foot-high feature has been mined for sand and gravel.
Most eskers and kames are composed of coarse, poorly sorted materials, a mixture of sedimentary textures ranging from silt and sand, up to large cobbles or boulders. As they were forming, flowing water deposited flat-lying beds of sand and gravel. Later though, when the ice-walls melted and the bedding collapsed, the bedding became contorted. While a stream was flowing in an ice-walled tunnel or valley, or water was flowing into a hole in the glacier (a “moulin”), cobbles and boulders fell from the melting ice into the water-lain deposits, into the stream or down the moulin. Some eskers have a nearly complete covering of boulders on their surfaces. Most kame or esker deposits contain so many cobbles they are not suitable for construction purposes. Some of them consist of materials suitable for rough fill work.