Superior Silica Sands— all with operations in Wisconsin—to liquidate their frac sand mines and declare bankruptcy. Geographic extent Minnesota Corresponding to the southeast geological region of Minnesota, the colloquial "Driftless Area" (though the whole region was glaciated) begins at about Fort Snelling. Starting as a narrow sliver against the Mississippi, it widens to the west as one goes south. The western boundary is the Bemis-Altamont moraine. Another more easily located reference to the western boundary is the approximate line of Minnesota State Highway 56. The upland plateau lies west of the incised tributaries to the Mississippi. The historic vegetation was mixed woodland, with occasional goat prairies on southwesterly facing slopes. In the western section is "an old plateau covered by loess [...] along the eastern border and pre-Wisconsin age glacial till in the central and western parts. The western portion is a gently rolling glacial till plain that is covered by loess in places." The counties involved include all or part of Dakota, Goodhue, Rice, Wabasha, Winona, Olmsted, Dodge, Houston, Fillmore, and Mower. Aside from the southeastern suburban sprawl of the Twin Cities, Rochester is the main urban area. Additional communities include Red Wing, Lake City, Winona, La Crescent, Chatfield, Lanesboro, Rushford, Houston and Caledonia. Glacial River Warren, in whose bed the Minnesota River now flows, entered the "Driftless Area" just downriver from present-day Minneapolis-Saint Paul, at Fort Snelling, over River Warren Falls, "an impressive 2700 feet (823 m) across and 175 feet (53 m) tall, over 10 times as wide as Niagara falls" (this has since receded to become Saint Anthony Falls). The region is characterized "by the absence of glacial drift deposits, the sculpted topography, and the presence of the ancient limestone immediately beneath the soil and in cliff outcroppings." The Minnesota Driftless Area did not reach the Twin Cities or any areas to the north or west of them; rather, the Twin Cities marked the edge of glaciation, with substantial terminal moraines overlying the region. The largest protected area is Richard J. Dorer Memorial Hardwood State Forest, which contains some state-owned land, but is mostly private, controlled by state conservation easements. Wisconsin Around 85% of the Driftless Area lies within Wisconsin, comprising much of the southwestern quarter of the state. The border is defined by the catchment of the Chippewa River on the north, and somewhat west (or east, depending on if the southwestern portion of Wisconsin's