Survey of Otter Tail County, Minnesota
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).
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
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
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.
Section across the Red River Valley
on the latitude of Breckenridge and
9,000 to 12,000 years ago, tremendous volumes
of meltwater 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
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,
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).