Formed 25,000 years ago by the Wisconsin Glacier, Great Swamp National Wildlife Refuge and Wilderness Area is what remains of a large lake, formed by the scouring action of glacial ice and ultimately drained by a river outlet once the glacier had receded. The bed of the lowland area was surrounded by small hills; as the low point in the watershed, it was fed by brooks and tributaries that brought silt. The silt filled portions of the basin, creating firmer, drier areas; over thousands of years a region of mixed swamps, grassy hummocks, and wooded areas was formed. The Great Swamp is still the source of the river, now called the Passaic, and is home to more than 244 species of birds, 39 species of reptiles and amphibians, 29 species of fish, 33 species of mammals, and more than 600 species of plants.