Geologic History and Geomorphology

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).

Right: Map showing the extent of Lake Agassiz at the times of formation of the Fergus Falls and Leaf Hills moraines (click for larger image)

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).

A more recent study, however, suggests that pre-late Wisconsin glacial ice that advanced from the Keewatin ice center that is now called the Winnipeg Lobe (formerly identified as the Wadena Lobe) was laid down before the drumlins by an older Rainy Lobe source from the northeast. Thus the Wadena Drumlin Field resulted from a progressive mixing of glacial till from a northeastern source, and ice lobes and flows molded the drumlins (Goldstein, 1985).

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 readvances 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 meltwaters flowing under the ice of the Wadena Lobe formed a few eskers. The large volumes of meltwater 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. Meltwater 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 meltwater channels flowing eastward. Water flowage was blocked in an area near Pillager, resulting in the formation of Glacial Lake Wadena. Meltwater and sediment were redirected along a meltwater channel toward Parkers Prairie. Eventually the blockage at the Pillager gap was broken, and meltwaters from the Des Moines and Wadena Lobes exited eastward through the gap into the Crow Wing River.