From Winona to La Crosse, the Mississippi River valley reaches its greatest depth as it cuts vertically through more than 240 meters of a sedimentary-rock plateau. Along this stretch, Highway 61 follows the narrow strip between the river and the steep bluffs that form the valley's western wall. The valley walls consist of sandstone and carbonate rock formed from sand and lime mud deposited about 500 million years ago in a warm, shallow sea that covered much of what is now North America. Lower, more sloping parts of the walls are made mostly of weakly cemented sandstone that erodes easily, while the upper walls are shaped by steep cliffs of erosion-resistant dolostone, a chemically altered limestone. As the Mississippi or a tributary cuts into the soft sandstone, sandstone rockfalls undercut the dolostone, which then breaks along vertical joints and leaves steep cliffs. Two especially prominent bluffs in the area, King's Bluff and Queen's Bluff, stand on the west side of the valley southeast of this site, with King's Bluff closer; both lie within Great River Bluffs State Park and are designated Scientific and Natural Areas by the Minnesota Department of Natural Resources for their unusual geology and rare biological communities. These bluffs are part of the Driftless Area, a region of deeply eroded stream valleys primarily east of the Mississippi River and covering southwestern Wisconsin, where glacial ice never passed during the last two million years and left no drift deposits. Even so, the landscape was covered with loess, a wind-blown tan rock dust carried from vegetation-free floodplains repeatedly loaded with very fine sediment by streams draining melting glaciers. A distinctive, fertile soil later developed in the loess and helps support the diverse and sometimes unique plant communities found on these bluffs.