slopes, sugar maple-basswood-oak forest on moister slopes, sugar maple-basswood forests in protected valleys and on north-facing slopes, wet prairies along the rivers and some mesic prairie on the floodplain farther back from the river. There were probably also oak forests that contained no sugar maple. Marsh and floodplain forests were also common on river flood plains. Prairie was restricted primarily to the broader ridge tops, which were unfavorable sites for trees due to thin soils and shallow bedrock, rapid drainage, and desiccating winds; all these conditions were also good for carrying fires across the landscape. Prairies also occurred on steep slopes with south or southwest aspect (see goat prairie). Natural fire, which has long been vigorously suppressed, was essential for the regeneration of such prairies. Evidence of ancient extinct ice age animals that once inhabited the Driftless Area has been discovered over the years. An example of extinct Pleistocene megafauna in the area is the Boaz mastodon, a composite skeleton of two separate mastodons found in the 1890s in southwestern Wisconsin. Although evidence exists that mastodons inhabited mostly coniferous spruce forests associated with the taiga biome, it is likely that most or all of the Driftless Area was at times covered by tundra and permafrost during periods of glacial maximums. The Midwest Driftless Area Restoration Effort is a multi-agency cooperative effort to restore the landscape. The main issues are water pollution from agricultural and animal runoff, and erosion. Many farmers in the region utilize contour plowing, strip cropping, and other agricultural practices to reduce soil erosion due to the hilly terrain. Water pollution is particularly critical in karsted regions such as this, in that it can degrade or destroy prime cold water fish habitat. Soil erosion presents the Army Corps of Engineers with a particular problem, in that it requires them to dredge the Mississippi River shipping channels to keep them open. Trout Unlimited is part of this effort, if only because of the superb cold-water streams the region supports. A symposium was held in October 2007 in Decorah, Iowa, "to share the results of research, management and monitoring work in the Driftless Area." The Nature Conservancy is also interested. The Driftless Area contains more than half of the world's algific talus slopes, a type of small, isolated ecosystem. These refugia create cool summer and fall microclimates which host species usually found further north. They contain at least one endangered