3.1 Geological History

The rocks highlighted in figure 3.1 are the oldest in the United States. They formed in Precambrian time, between 4.5 billion and 560 million years ago. The oldest Precambrian rocks shown here are about 2.6 billion years old, meaning that they formed during the Archean Era. Even older rocks are exposed in Canada ; the Superior Upland in Minnesota and Wisconsin is a younger part of this ancient "core" of North America, called the Canadian Shield.

Figure 3.1 Precambrian Rock Formations.

The Canadian Shield is a large craton in eastern and central Canada and adjacent portions of the United States , composed of bare rock dating to the Precambrian Era. It is also called the Precambrian Shield, or Laurentian Shield, or Laurentian Plateau.

Such a large area of exposed, old rock requires some explanation. The current surface expression of the Shield is one of very thin soil lying on top of the bedrock, with many bare outcrops. This arrangement was caused by severe glaciation during the last ice age, which covered the Shield and scraped the rock clean. The multitude of rivers and lakes in the entire region is caused by the watersheds of the area being so young and in a state of sorting themselves out with the added effect of post-glacial rebound. The Shield was originally an area of very large mountains and much volcanic activity, but over the millennia the area was eroded to its current topographic appearance of relatively low relief.

Fig 3.3 Minnesota Rock Ages

3.1.1 The Ice Age

An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the continental ice sheets, polar ice sheets and mountain glaciers (glaciation). Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age because the Greenland and Antarctic ice sheets still exist. More colloquially, when speaking of the last few million years, ice age is used to refer to colder periods with extensive ice sheets over the North American and European continents: in this sense, the last ice age ended about 10,000 years ago.

Post-glacial rebound, sometimes called continental rebound, isostatic rebound or isostatic adjustment, is the rise of land masses that were depressed by the huge weight of ice sheets during the last ice age, through a process known as isostatic depression. It affects Canada , and the Great Lakes of Canada and the United States.

Fig. 3.4. During glaciation.

Fig. 3.5. After glaciation.

By the end of the last ice age, much of North America was covered by ice sheets up to 3 km thick. The enormous weight of this ice caused the crust to sink into the fluid mantle. At the end of the ice age when the glaciers retreated, the removal of the weight from the depressed land led to a rapid uplift due to the buoyancy of crustal material relative to the mantle. Due to the extreme viscosity of the mantle, it will take many thousands of years for the land to reach an equilibrium level.

Studies have shown that the uplift has taken place in two distinct stages. The initial uplift was rapid, proceeding at about 7.5 cm/year. This phase lasted for about 2000 years, and took place as the ice was being unloaded. Once de-glaciation was complete, uplift slowed to about 2.5 cm/year, and decreased exponentially after that. Today, typical uplift rates are of the order of 1 cm/year or less, and studies suggest that rebound will continue for about another 10,000 years. The total uplift from the end of deglaciation will be about 400 m.

3.1.2 Lake Agassiz

Continental glaciation formed the landscapes of Otter Tail County . Bedrock in Otter Tail County is covered by 200 to more than 400 feet of Wisconsin glacial deposits. Glaciers moving as lobate rivers of ice advanced across the survey area many times and from several different directions. The present landscapes of Otter Tail County began to take shape during the early and middle parts of the Wisconsin Glaciation. The geomorphic landforms in Otter Tail County are drumlin fields, stagnation moraines, till plains, pitted outwash plains, kame moraines, and glacial lake plains (Hobbs and Goebel, 1982).

The Wadena Drumlin Field fans out across Wadena, Todd, Cass, Hubbard, Becker, and Otter Tail Counties of Minnesota . This geomorphic area represents the oldest landscape in Otter Tail County . Radiocarbon dating of organic silts and lake sediments suggests that the drumlins are about 30,000 to 60,000 years old (Wright, 1972).

In Otter Tail County , the drumlins are in the northeast corner and near the Todd and Wadena County lines south of the Leaf River . The Wadena Drumlin Field consists of a series of low, smooth, elongated oval hills and broad, nearly level depressions that are oriented from east to west. The fanlike shape of the drumlin field suggests that ice moved from the northeast.

There are two main theories regarding the formation of the drumlins. Studies of pebble lithology and carbonate content suggest that glacial till moved southeastward from the Winnipeg Lowlands into northern Minnesota . This movement was diverted by another glacial lobe advancing from the east (Wright, 1962).

When the Wadena or Winnipeg Lobe advanced from the east and northeast across the county, it formed the core of the Alexandria Moraine and the Wadena Drumlin Field. Later re-advances of the Wadena Lobe occurred about 20,000 years ago.

The coarse-loamy till of the Wadena or Winnipeg Lobe is characterized by less than 18 percent clay and more than 50 percent sand ( Anderson , 1976). The Alexandria Moraine Complex consists of stagnation moraines. These stagnation moraines formed at the outer edges of a glacial lobe. There are also some smaller areas of stagnation moraine in the eastern half of the county. Stagnation moraine landscapes have a complicated pattern of soil materials. Although they are mostly made up of glacial till, some are local deposits of outwash and water-laid sediments. The moraines are typically the highest in elevation, have the greatest relief, and are commonly hilly.

There are many small to large ice-block basins in stagnation moraines that now contain lakes or marshes. Ice-walled lakes formed when pits in the stagnant ice on the Alexandria Moraine filled with water-laid sediments. Later, as the ice melted, the surrounding landscape collapsed and the lake bottom became what is now the hilltop (Clayton and Cherry, 1967).

The stagnation moraines in Otter Tail County were formed by ice advances from both the Wadena or Winnipeg Lobe and the Des Moines Lobe. The Altamont Moraine in Otter Tail County is the portion of the Alexandria Moraine that was overridden by the Des Moines Lobe glacial drift. About 14,000 years ago, the Des Moines Lobe advanced to the south across Manitoba , where it incorporated limestone rocks. As the glacial lobe moved south along the Red River Lowland, shale-rich materials derived from Cretaceous rock were mixed into the till fabric (Sackreiter, 1975). The Des Moines Lobe continued to spread east into Otter Tail County onto the Alexandria Moraine and southeast along the moraine. Before it retreated, the Des Moines Lobe left behind the Big Stone Moraine, which is also referred to as the Fergus Falls Till Plain (University of Minnesota, 1969). The fine-loamy till of the Des Moines Lobe is characterized by more than 18 percent clay, typically less than 50 percent sand, and a high content of shale.

The Henning Till Plain was formed behind the advancing front of the glacial ice of the Wadena or Winnipeg Lobe and the Des Moines Lobe. The till plain has generally low or moderate relief and is typically gently undulating. In places, especially south and west of New York Mills, the till was deposited over older outwash deposits. Thus, in some areas on the Henning Till Plain, the till ranges from about 3 feet to more than 10 feet thick over outwash. Also on the till plain, glacial melt waters flowing under the ice of the Wadena Lobe formed a few eskers. The large volumes of melt water pouring east and south off the Wadena and Des Moines Lobes left extensive outwash plains. This outwash area formed the Detroit Lakes Pitted Outwash Plain in the central part of the county. Pitted outwash plains are characterized by many small to large ice-block basins that now contain lakes or marshes. The areas of the Detroit Lakes Pitted Outwash Plain adjacent to the Alexandria Moraine are described as kame moraines in the county.

The landforms and topography of the kame moraine are similar to those of the stagnation moraine, except that the parent material is outwash. Melt water sediments flowing eastward from the Henning Till Plain and southward from the Itasca Moraine Complex formed the Park Rapids-Staples Outwash Plain. The Redeye and Leaf Rivers were major melt water channels flowing eastward. Water flowage was blocked in an area near Pillager, resulting in the formation of Glacial Lake Wadena. Melt water and sediment were redirected along a melt water channel toward Parkers Prairie. Eventually the blockage at the Pillager gap was broken, and melt waters from the Des Moines and Wadena Lobes exited eastward through the gap into the Crow Wing River. 

About 9,000 to 12,000 years ago, tremendous volumes of melt water accumulated to form Glacial Lake Agassiz. Lake Agassiz was more than 360 feet deep and covered more than 120,000 square miles in Minnesota , North Dakota , and Canada. It was formed when the normal northward flow of water in the Red River Valley was blocked by the Des Moines glacial ice lobe. As water levels rose higher, Glacial Lake Agassiz developed an outlet to the south near Browns Valley. A succession of beach ridges formed, marking stable lake levels. The Herman Beach marks the highest stable level of Lake Agassiz. It runs along the eastern edge of the Lake Agassiz Plain in Otter Tail County. When the Des Moines glacial lobe retreated, the water in the Red River Valley could once again flow north and Lake Agassiz drained away (Elson, 1967).

The most recent deposits in the survey area are not glacial in origin. They consist of alluvial sediments on flood plains and lakeshores. Organic material and limnic sediments in lakes and depressions are estimated to have accumulated about 4,000 to 5,000 years ago (Norton, 1982).

3.1.3 Formation of the Pelican Group of Lakes

The Pelican group of lakes was formed by the last retreating glacier of the Red River Lobe about 10,000 years ago. The glacier left behind primarily sands and loamy sands 50-350 feet deep. Since the retreat of the glacier the watershed characteristics have been in a state of imbalance. The flow of water and formation of wetlands has been changing for the last 10,000 years. This is due to the post-glacial rebound of the area. The chaotic or non-steady state, characteristics of the watershed is why continued, non-interrupted, monitoring is so important. While large changes in characteristics will take thousands of years, minor changes can be induced naturally in less than one year; however, man can influence changes in characteristics overnight.

next page