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Nelson (1986) gives a succinct description of the geology of the SNG region. The North Unit of the SNG is situated on a geologic unit known as the Sheyenne River Delta. Brophy (1967) has described the geology of this unit in more detail. The delta formed approximately 12,000 to 10,000 years ago at the end of the Wisconsin Glacial Period as the Sheyenne River carried overflow from Glacial Lake Souris to Glacial Lake Agassiz. Thickness of the sediments making up the delta varies from 50 ft at the western edge to 170 to 180 ft at the eastern margin. The sediment was sorted according to the river's energy, leaving the coarser sands to the west and gradually depositing the finer sands as it traveled east (Bluemle 1975). The sandy deposits were reworked by wind, after Glacial Lake Agassiz drained and before vegetation was established, forming the dunelike formations that are characteristic of the area (Bluemle 1979). Today the area is locally known as the "Sandhills." The area is generally of low relief, and much of the area is nearly level. However, a significant part of the area is also gently undulating to hummocky and sharply choppy.

The deltaic deposits are divided into 3 main units. The deepest deposits are silt interbedded with clay and sand. Above these deeper deposits is a middle unit of well sorted sand. The surface unit composed of wind blown sand which is usually less than 10 ft thick, except the high dunes in the choppy areas. The lower unit of the deposits is nearly impervious, resulting in a relatively high water table, and forming what is known as the Sheyenne Delta aquifer. Quite generally the water table is about 10 ft below the surface (Baker and Paulson 1967). This condition, along with the an average annual precipitation of about 20 inches, results in a large portion of the delta area supporting a tallgrass prairie ecosystem.

The parent material for the soils of the SNG resulted from the wind erosion of deltaic and lacustrine deposits. Figure 1 shows the toposequence of the main soil series within the delta landscape (Nelson 1986).
The soils on the convex slopes tend to be drier, thus providing less favorable conditions for plant growth. Soils on the nearly level to concave positions are moister allowing greater vegetative production. During drought the upper profile dries out, especially on the convex slopes, resulting in the loss of vegetative cover and the susceptibility of some areas to wind erosion. Careful management to maintain adequate vegetative cover and prevent wind erosion is a prime concern of federal land managers and private owners alike.

Legend for Habitat Type:

Bouteloua gracilis / Stipa comata habitat type   - Bogr / Stco h.t. Andropogon gerardi / Andropogon scoparius  - Ange / Ansc h.t. Carex lanuginosa / Calamagrostis inexpansa  - Cala / Cain h.t. Stipa comata / Carex heliphila                         - Stco / Cahe h.t. Andropogon hallii / Calamovilfa longifolia     - Anha / Calo h.t. Bouteloua hirsuta / Carex heliophila               - Bohi / Cahe h.t.

Figure 1. Toposequence diagram indicating the location of the habitat types (h.t.) associated with typical topo-edaphic conditions at the Sheyenne National Grassland (from Nelson) 1986).

Although Weaver and Clements (1938) described the indigenous vegetation type of eastern North Dakota as tallgrass prairie, the vegetation of the SNG is a mixture of tallgrass prairie, mixed grass prairie, forest and woodland communities intermixed with agricultural cropland (Barker 1974, Manske and Barker 1988, Nelson 1986, Wanek 1965). Barker (1974) presented a general description of the native vegetation of the sandhills region by describing 9 plant communities. The 9 plant communities recognized by Barker are: sedge-cattail-willow, elm-basswood, green ash-boxelder, quaking aspen-cottonwood, bur oak, bur oak savanna, mixed grass prairie, tallgrass prairie, and sedge meadow. He listed the characteristic species for each plant community and described the distribution of these communities as determined primarily by soil-moisture relationships and topography. The sedge-cattail-willow community develops along the margins of the water courses in high soil moisture areas. The elm-basswood community is found on the first alluvial terrace along the Sheyenne River. The plants of this community have a very high water requirement and do not extend far from the river. A green ash-boxelder community is found farther away from the river where slightly lower soil moisture conditions exist. The quaking aspen-cottonwood community occurs where the green ash-boxelder community has been disturbed and in scattered pockets in other plant communities. A dense woodland of bur oak forms at the top of the river valley where there is still sufficient soil moisture. As the soil moisture decreases, the bur oak community opens into a bur oak savanna with a mixed grass prairie community understory. The oaks soon drop out and the vegetation consists primarily of grassland communities as the soil moisture is lower and the topographic relief is decreased from choppy sandhills to undulating low hills to relatively level land. The mixed grass prairie community is found on top of the undulating hills. The tallgrass prairie community is found at the margins of the hills. The sedge meadow community is found in the depressions, between the hills, where the soil moisture is higher.

Manske (1980) used the present vegetation of the SNG in recognizing 11 habitat types. These habitat types were based on areas of distinctly similar topography, soil characteristics and vegetative composition. Eight of the habitat types were comprised of native vegetation and three were replacement communities. The habitat types which exhibited closely related characteristics and distribution were grouped into 4 habitat associations: Hummocky Sandhills, Deltaic Plain, Choppy Sandhills, and River Terrace. Manske stated that because the habitat types exhibited such complex, intricate patterning and were quite small at individual locations, their groupings into habitat associations should be used as map units and in general management considerations. Manske constructed a map of habitat associations by combining information of vegetation, soils and topography (Figure 2).

Manske (1980) determined the acreage of each habitat type and habitat association (Table 1).

Figure 2. Habitat associations of the Sheyenne Naitonal Grasslands (Manske 1980) and prairie grouse display grounds (from Manske and Barker 1988).

Table 1. The estimated surface area of the habitat types on the Sheyenne National Grasslands. (Estimations made by use of a dot grid). From Manske (1980).

The Hummocky Sandhills Habitat Association comprises approximately half of the total (public + private lands) SNG area (65,494 ac or 50.16%). Manske (1980) divided this habitat association into 4 habitat types. The Upland Habitat Type occurs on the summit and shoulder slopes of each hummock and has a combined area of 34,389 acres (26.34%). The dominants are Bouteloua gracilis - Stipa comata - Carex heliophila of the mixed grass prairie community. The Midlands Grassland Habitat Type occurs on the back and foot slopes of each hummock and totals 16,558 acres (12.68%). The dominants of the tallgrass prairie community are Andropogon gerardi - Andropogon scoparius - Panicum virgatum. The Lowland Grassland Habitat Type occurs on the foot and toe slopes and has a total area of 12,737 acres (9.67%). The dominants of the sedge meadow community are Carex lanuginosa - Calamagrostis inexpansa - Juncus balticus. The amount of Cropland Habitat Type on this association is quite small, 1,810 acres (1.39%) generally found on what would be midland habitat type. These support mostly maize and alfalfa crops. There is also a very small amount of planted tree shelterbelts. Newell (1987) recognized an upland shrub community in this association. This shrub community, most often dominated by buckbrush (Symphoricarpos occidentalis) with an understory of associated upland or midland grasses and forbs, usually occurs on the shoulders of the hummocks and can extend to the foot.

The Deltaic Plain Habitat Association consists of 38,761 acres (29.69%). The topography is nearly level and has a high water table. This association is divided into 3 habitat types. The Midland Grassland Habitat Type occurs on areas that are slightly elevated and totals 14,476 acres (11.09%). The tallgrass prairie dominants are Andropogon gerardi - Andropogon scoparius - Sorghastrum nutans. A small amount of mixed grass prairie (< 15 ac or 0.01%) dominated by Bouteloua gracilis - Stipa comata occurs on areas of greater relief. The Lowland Habitat Type is found in the slight depressions, having a combined area of 5,387 acres (4.13%). The dominants of this sedge meadow community are Carex lanuginosa - Calamagrostis inexpansa - Carex spp. Newell (1987) recognized an additional native lowland community which he termed the Lowland II Community, dominated by Spartina pectinata, Calamagrostis inexpansa, Poa pratensis and Carex spp. He noted this community type as being less common than the sedge meadow community. The Cropland Habitat Type forms a large part of this association (18,898 acres or 14.47%) because of the level topography and fertile soil. The main crops grown on these areas are corn, alfalfa and sunflowers. Some 402 acres (3.08%) of tree shelterbelts are a part of this habitat type.

The Choppy Sandhills Habitat Association occurs on 19,170 acres (14.68%) of the SNG. Manske divided this habitat association into 2 habitat types. The Upland Woodland Habitat Type is found on the slopes and depressions of the choppy topography and totals 12,269 acres (9.40%). The dominant trees are Quercus macrocarpa - Populus tremuloides - Fraxinus pennsylvanica. These exist as dense groves to scattered individual trees. The Open Grassland Habitat Type occurs between the dense groves and comprises a total area of 6.901 acres (5.29%). The dominants of the open grassland are Bouteloua gracilis - Carex heliophila - Sporobolus cryptandrus.

The River Terrace Habitat Association occurs along the Sheyenne River and its tributaries. It totals 7,135 acres (5.46%). This association is divided into 2 habitat types. The Riparian Forest Habitat Type occurs throughout the river terrace and valley escarpment and totals 5,710 acres (4.37%). The dominants of this forest community are Tilia americana - Ulmus americana - Fraxinus pennsylvanica. Small areas of sedge-cattail-willow wetland communities occur in the oxbows and along the river channel. The Cropland Habitat Type is found on areas cleared of forest vegetation, and totals 1,425 acres (1.09%). The main crops grown in this habitat type are corn, sunflowers and alfalfa.