Great River Bluffs

Great River Bluffs. . From Winona to La Crosse, the Mississippi River valley displays its greatest depth as it extends vertically through more than 240 meters of a sedimentary-rock plateau. Here, Highway 61 follows the narrow strip between the river and the steep bluffs that mark the valley's western wall. The valley walls are composed of sandstone and carbonate rock, which 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. The lower, more sloping parts of the valley walls are composed mostly of weakly cemented sandstone, which erodes easily. On the upper parts of the walls, steep cliffs shape the bluffs. The cliffs are composed of dolostone, a chemically altered limestone that is resistant to erosion., Bluffs are formed as the Mississippi or a tributary cuts into the soft sandstone, initiating sandstone rockfalls that undercut the dolostone. The dolostone then breaks along vertical joints, leaving steep cliffs. Two of the most prominent bluffs in the area, King's Bluff and Queen's Bluff, are visible southeast of this site on the west side of the valley. King's Bluff is the closer one. Both are 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 within the "Driftless Area," an area of deeply eroded stream valleys primarily east of the Mississippi River and covering southwestern Wisconsin. During the Ice Age of the last two million years, glacial ice never passed over and leveled this area, and no drift, or glacially carried sediment (clay, silt, sand, gravel, and boulders), was deposited here. However, the landscape before you was blanketed with a layer of loess, a wind-blown, tan-colored rock dust. This dust was carried by winds from floodplains still bare of vegetation, which were repeatedly loaded with very fine sediment by streams that drained melting glaciers. Today, a distinctive and fertile soil has developed in the top of the loess, which helps to give rise to the diverse and sometimes unique plant communities found on these bluffs.