The 2,492-acre Waterfall Glen Forest Preserve in Darien is one of the most ecologically impressive parcels of open space within the Forest Preserve District of DuPage County, if not northern Illinois. The ridges and scattered potholes that make up the topography of Waterfall Glen — and most of the county — are the result of one geological event, the Wisconsin Glacier. In fact, the entire Des Plaines River Valley was formed by glacial meltwaters and drainage from a postglacial lake that covered most of the Chicagoland area. Today, Waterfall Glen’s prairies, savannas, and oak-maple woodlands contain 740 native plant species, 75 percent of all the plants known to grow naturally in DuPage County. As a result of this rich variety, over 300 species of mammals, birds, fish, amphibians, and reptiles and another 300 of invertebrates use the forest preserve either year-round or during their migrations.