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Glaciation

About 7 million years ago, the subtropical climate of what is now the Dakotas began to change to a continental climate of cool winters and warm summers (Bluemle 1977). During the Pleistocene Epoch that followed, a succession of great ice sheets inched southward from Canada and covered most of the Dakotas (Figure 4). These huge glaciers transported vast quantities of rock and soil. Large amounts of local silty and clayey bedrock outcrops were also pulverized and added to the mixture, forming glacial drift or "till" that was deposited as sediment across most of the area glaciated. These deposits may be >250 m thick in places and are dotted with shallow basins left by the scouring or shearing action of the glaciers, or from the collapse of ice blocks left to melt in the deposits after the glaciers retreated. According to Flint (1955), many basins in South Dakota were formed when glacial drift materials blocked ancient bedrock valleys. Additional basins were formed by materials deposited by water flowing from melting glaciers. Thus, all the basin wetlands discussed in this report occur in a geologically young landscape.

Figure 4. Extent of Pleistocene glaciation in North America in relation to North and South Dakota (modified from Bluemle 1977).

Stewart and Kantrud (1973) estimated that 93% of the area and 94% of the number of wetlands in the Prairie Pothole Region of North Dakota were composed of natural basin wetlands, whereas the remainder were mostly streams and oxbows, stockponds and dugouts, reservoirs, road ditches and drainage channels, and minor types such as sewage lagoons. These figures show the importance of glaciation on the composition of wetlands in the region.

Physiographic Divisions

Geobotanists have traditionally divided the northern grasslands into two large areas called the Great Plains and Central Lowland (Fenneman 1931) (Figure 1). The more arid Great Plains supports native grassland that is shorter than that in the moister Central Lowland to the east. The Prairie Pothole Region of the Dakotas encompasses portions of both these areas.

The Great Plains portion of the Prairie Pothole Region of the Dakotas contains a single physiographic division, the Missouri Coteau (Figure 3). This division is approximately 52,000 km² in area. The entire west slope of the Missouri Coteau drains to the Gulf of Mexico via the Missouri River.

By far the largest number and area of basin wetlands in the Dakotas occurs in the Central Lowland. Most of this land mass drains either to Hudson's Bay (North Dakota) or the Gulf of Mexico (South Dakota). Within the Central Lowland lie six major physiographic regions (Figure 3). These are, in decreasing order of area, the Glaciated Plains (921,000 km²), Prairie Coteau (15,2OO km² ), Dakota Lake Plain (5700 km² ), Souris Lake Plain (3600 km²), Devil's Lake Plain (1400 km²), and Turtle Mountains (1200 km²).

All seven physiographic divisions contain a variety of glacial or postglacial landforms. Those landforms that contain relatively numerous wetland basins will be discussed in the following section.

Glacial Morainic Landforms

Most natural basin wetlands in the Prairie Pothole Region of the Dakotas are found in five glacial morainic landforms (Bluemle 1977). Moraine means any materials deposited directly by glaciers.

Ground moraine. This is the predominant glacial landform of the Glaciated Plains (Figure 3), and can be recognized by a gently rolling landscape with numerous shallow saucer-shaped depressions, but few hills or deep cup-shaped depressions (Bluemle 1977). This landform occurs where moderate amounts of glacial till were deposited at the base of a moving glacier and by collapse from within the glacier when it finally melted.

Washboard moraine. This form appears as small areas of irregularly spaced ridges of material thought to have been carried upward through the ice along shear planes parallel to the edge of the glacier (Bluemle 1977). Small basins are numerous in washboard moraine. This landform is mostly found in association with ground moraine in the Glaciated Plains.

Thrust moraine. This is perhaps the most spectacular glacial landform, as it is the result of large-scale glacial shearing that moved blocks of land up to 20 km in area for short distances (Bluemle 1977). The "hole" left by these blocks commonly resulted in a large lake, whereas the hilly blocks often contain numerous small but relatively deep basins. Most thrust moraine is found in the Glaciated Plains.

Terminal moraine. This form resulted when glacial till was deposited at the edge of a glacier while the ice margin was melting back at about the same rate as the ice was moving forward (Bluemle 1977). Till is a general term for the mixture of materials ranging in size from clay particles to boulders of many tons that were pushed forward by and carried on top of advancing glaciers. Terminal moraines are most common in the Glaciated Plains, but also occur in the Missouri and Prairie Coteaus (Figure 3). These moraines are commonly 2-15 km wide and 5-90 km long. Basins in terminal moraine are highly variable in size, depth, and density.

Dead-ice moraine. This form is responsible for some of the most rugged glacial topography in the Dakotas: it formed when glaciers advanced over steep escarpments. Shearing action carried material into and on top of the glacier (Bluemle 1977). This insulated the underlying ice, which took several thousand years to melt and collapse. When the overlying materials slumped and slid, thousands of basins of all shapes and sizes were formed. Dead-ice moraine is the most common landform in the Missouri Coteau, Turtle Mountains, and Prairie Coteau. Smaller amounts of dead-ice moraine occur in the Glaciated Plains.

Glacial Meltwater Landforms

Three glacial meltwater landforms contain significant numbers of basin wetlands. These landforms were created by water from melted glacial ice, including precipitation on glaciers as they were melting (Bluemle 1977).

Glacial outwash plain. This form consists of sand and gravel that was deposited by water flowing from melting glaciers. In some of these broad plains are numerous large lakes, whereas in others are large numbers of very shallow depressions. This landform is mostly found in the Glaciated Plains.

Collapsed glacial outwash. This form resulted when glacial outwash was deposited on stagnant ice. When the ice melted, the sand and gravel slumped to form an irregular hilly surface with numerous wetlands, many of them large, shallow lakes (Bluemle 1977). Most collapsed glacial outwash occurs in the Missouri Coteau, but several large examples can also be found in the Glaciated Plains. Many of the largest basins in North Dakota are in collapsed glacial outwash, and many of the lakes formed in these basins are highly saline.

Glacial lake plains. These areas occur today where lakes of glacial meltwater stood for hundreds or thousands of years (Bluemle 1977). Glacial Lake Agassiz was the largest of such lakes in what now are the Dakotas. This giant water body once occupied a vast area encompassing most of southern Manitoba and Ontario, as well as huge areas of Saskatchewan and northern Minnesota. Nearly all the glacial lakes vanished when the glaciers receded and outlets were established. Topography is very flat in these areas, and wetland basins are very shallow. However, near the center of some glaciated lake plains, a few large remnant basins are still present. Most glacial lake plain in the pothole region occurs in the Dakota, Souris, and Devil's Lake Plains (Figure 3), but small areas also can be found in the Glaciated Plains and Missouri Coteau. In the latter area and the Turtle Mountains, there are also some small areas where glacial lakes flooded stagnant ice for shorter periods, resulting in hilly topography with numerous basins when the stagnant ice finally melted.