The work of John Bluemle PhD

11-GLACIERS IN NORTH DAKOTA – PART TWO

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.

till, fluvial deposits, North Dakota, geology

Fig. 11-A. This photo shows till (on top) overlying banded gravel (the materials at the bottom of the photo have fallen from above). The tan till at the top was deposited by a glacier flowing over fluvial (water-lain) deposits, which were likely deposited by water flowing from the glacial ice. The contact between the base of the till and the top of the gravel, the bedding of which is truncated, is remarkably sharp. These materials are old; deposited by a pre-Wisconsinan glacier. The till contains abundant pieces of lignite coal, so we know that the glacier that deposited it flowed over lignite-bearing Tertiary rocks to the northwest. Till deposited by glaciers flowing from the east or northeast contain little or no lignite. McLean County. Photo-scan. 1978.

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.

Wisconsinan till, till on bedrock, North Dakota, geology

Fig. 11-B. The darker material on top, with numerous vertical iron-stained fractures is glacial material (till), likely Early Wisconsinan age (about 70,000 years old). Notice the pebbles in the glacial sediment, an easy way of identifying till. The till lies directly on top of bedrock of the Sentinel Butte Formation, fine-grained siltstone, about 57 million years old. The contact between the bedrock and the overlying till is known as an “unconformity,” meaning there is a substantial break in the geologic record, both in time and manner of deposition. In this case, the bedrock beneath the till is about 800 times older than the till on top. McLean County. Photoscan. 1965.

 

 

 

 

 

 

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.

glacial lake sediments, North Dakota, geology

Fig. 11-C. Evenly bedded sediments of Glacial Lake Agassiz, south side of Mayville, Traill County. The light-colored beds of silt are about 5 inches thick. They are separated by thinner dark bands. The light-colored bands were deposited on the floor of the lake during a single summer season, the dark bands during a winter. During the summer, steams flowing into the lake carried large amounts of silt, which accumulated in thicker layers on the lake floor. In the winters, the amount of incoming sediment was much less. The layers shown here record about 15 years of Lake Agassiz’s history. Photoscan. 1978.

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.

gravel deposit, North Dakota, Griggs Co.

Fig. 11-D. Gravel exposed in a pit four miles west of Hannaford, Griggs County. This gravel is part of an esker deposit. Most esker gravels tend to be less well-sorted than this one, with inclusions of till, boulders, etc. Layers of coarser materials were deposited by fast-moving water; finer materials by slower moving water. Photoscan. 1969.

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.

geology, North Dakota, paleosols

Fig. 11-E. The two horizontal black lines are buried soils—paleosols—in alluvium along the Cannonball River in Sioux County. These two buried soils are unusually level and therefore easy to recognize. The soils formed on river alluvium at times the river was not depositing sediment in that area, when the river was not building its bed. I am unsure how much time is represented by each of the two paleosols – perhaps it took a few thousand years for each soil to develop (the soils have not been dated). Nor do I know how long it took for the river to deposit the alluvial sediment during the three periods of deposition shown (the light-colored silty sediment below the lower paleosol and above each of the paleosols). Deposition of the sediment might have been much quicker than the time it took for the soils to form. This area was probably not glaciated during the most-recent glaciation. Photoscan. 1978

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.

glacial ice, geology, North Dakota

Fig. 11-F. Debris-covered glacial ice in Alaska. When thick, debris-laden glacier melts, the material that was within the ice becomes concentrated on the surface of the remaining ice. As the debris cover becomes thicker, it becomes an increasingly effective cover of insulation, causing the remaining ice to melt more and more slowly. During Late Wisconsinan time, about 14,000 years ago, debris-covered glacial ice like that shown here covered the Missouri Coteau and Turtle Mountain. It may have taken as long as 3,000 years for the insulated ice to melt. During that time, forests grew on top of the slowly melting glacier. The resulting topography is referred to as “dead-ice-moraine.”
In the distance (top of this photo), forest can be seen growing on the debris-covered glacial ice. Photoscan UND Geology Dept. 1962.

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.

 

paleolsol; McLean County, Deadman Till, geology

Fig. 11-G. This exposure of glacial deposits is along Lake Sakakawea near Riverdale. Two tills are being eroded and exhumed by wave action at the lake shore. The upper till (farthest back and more brown in color) is early Wisconsinan in age. The lower till, which is pre-Wisconsinan in age, is much older and harder than the upper one. For this reason, the upper till is being removed much faster than the lower ones. This leaves the older till surface stripped of its former covering of younger till. Photoscan. 1978

 

Now Available
North Dakota’s Geologic Legacy Our Land and How It Formed
North Dakota's Geologic Legacy 12-10-15-412
Purchase from Amazon Purchase from NDSU Press
Recent Comments
    Archives
    Categories