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Hill prairies, comprised of a mixture of wildflowers, grasses and shrubs, occur on steep, southwestern-facing bluff systems, mainly along major river valleys. Growing conditions for plants are tougher here than elsewhere due to the intense sunlight, rapid drainage, and drying southwesterly winds, which prevail during the midwestern summers. Periodic fires are necessary to maintain the hill prairies; without fires, trees and shrubs take over from the prairie vegetation, despite the harsh growing conditions. This small, remnant hill prairie, shown in the photograph taken at Crevecoeur Nature Preserve, is burned off every few years; even so, the surrounding forest is barely being kept at bay. The Illinois River is just beyond the base of the river bluffs.