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Stable and partially active dune fields, dominated by parabolic dunes, are widespread over the subhumid to semiarid northern Great Plains of southern Canada and the northern United States (Fig. 15). Although winds in the northern Great Plains are among the strongest in the world (compared to other areas where eolian sand is found), most dunes are presently inactive because of relatively high ratios of precipitation to potential evapotranspiration, which has the dual effects of increasing moisture content within dunes and maintaining a vegetation cover (Wolfe, 1997). Dune fields in the region are smaller than those in the central and southern Great Plains, as most are derived from finite supplies of glaciofluvial or glaciolacustrine sediments deposited during the last deglaciation (David, 1971; Wolfe et al., 1995, 2000; Wolfe, 1997; Muhs et al., 1997b; Muhs and

Wolfe, 1999). Although some dune fields in Canada appear to have been generated from glaciofluvial and glaciolacustrine sediments by northeasterly glacial an-ticyclonic winds as the ice receded (David, 1981, 1988), stratigraphic and geochronologic studies indicate that many northern Great Plains dunes are not relict features from the last deglaciation. The last episodes of eo-lian activity were during the late Holocene, and several localities show evidence of having been active in the past millennium (Fig. 16), as is the case with the central and southern Great Plains. Although several dune fields in the northern Great Plains of Canada and the northern United States show evidence of multiple episodes of activity in the past few thousand years, as yet, there is too little evidence to ascertain whether there is regional synchroneity of dune sand movement.

FIGURE 13 Stratigraphy and ages of selected eolian sand sections in Nebraska, Colorado, and Kansas that show evidence of late Holocene eolian activity. Data are from Stokes and Swinehart (1997), Muhs et al. (1997b), Madole (1994), and Arbogast (1996). Locations are shown in Fig. 7.

Historic accounts indicate that at least four dune fields had some degree of activity in the mid-nineteenth century before major European settlement of the region took place (Muhs and Wolfe, 1999). Orientations of stabilized parabolic dunes indicate late Holocene paleowinds from the west or northwest, similar to dune-forming winds of the present (Fig. 15).

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