very deep below the surface, and is overlain by large amounts of sediment. As home to the formation of a substantial portion of the gorge of the Upper Mississippi, this enormous quantity of sediment goes down at least under the present riverbottom at the confluence of the Wisconsin River. In contrast, as the Mississippi exits the Driftless Area "between Fulton and Muscatine, [... (Pool 13)], it flows over or near bedrock." "The course of the upper Mississippi River along the margin of the Driftless Area of southeastern Minnesota is believed to have been established during pre-Wisconsin time, when a glacial advance from the west displaced the river eastward from central Iowa to its present position." Other rivers affected by this geologic process are: In Wisconsin, the Chippewa, Trempealeau, La Crosse, Black, Baraboo, Pecatonica, and Wisconsin Rivers, along with the Wisconsin River's tributary, the Kickapoo River; In Minnesota: the Whitewater, Cannon, Zumbro, and Root rivers; In Iowa: the Upper Iowa (and Paint Creek), Yellow, Turkey, and Maquoketa rivers; In Illinois: the Apple River and the Galena River (a.k.a. the Fever River). Although lying just to the north of the Driftless Area, the Saint Croix in Wisconsin and Minnesota is another important river that affected the area, as it was the outlet for Glacial Lake Duluth, forerunner to Lake Superior, when the eastern outlet was blocked by the continental ice sheet. All major rivers in and adjacent to the Driftless Area have deep, dramatic canyons giving testimony to the immense quantity of water which once surged through them as a result of the nearby melting Glaciers associated with the miles-high Ice sheets during recurring Ice ages. Other examples include the Wisconsin River, which drained Glacial Lake Wisconsin, and Glacial River Warren (whose bed is now occupied by the Minnesota River), which drained the colossal Glacial Lake Agassiz. There was ample water to dig a very deep, hundreds-of-miles-long gash into the North American bedrock where the Upper Mississippi River now flows. Ecosystem The climate is humid continental, displaying both the cool summer and warm summer subtypes as one travels from north to south. The United States Department of Agriculture has the region falling mainly in zone 5a, with the northern fringe being 4b. A few patches in Wisconsin are 4a. Prior to European settlement in the 19th century, the vegetation consisted of tallgrass prairie and bur oak savanna on ridgetops and dry upper