A National Natural Landmark, this 167-acre preserve is a mosaic of remnant prairie communities. The high quality wetlands and prairie are located on the former lacustrine plain of glacial Lake Chicago, resulting in a variety of clay and sandy soil deposited by the receding lake. This site includes a tallgrass black soil prairie, now rarely found in Illinois, whose rich soil has been plowed for agriculture. Under the influence of diverse substrate and water conditions, a wide range of plant species grow in the diverse habitats of this site.

The Gensburg-Markham Prairie is a large natural area of wet and dry mesic prairie, sand prairie, and sedge meadows characteristic of the Chicago Plain in the northeastern portion of the Moraine Natural Area. The Chicago Plain was formed when the Chicago Glacial Lake retreated at the end of the Wisconsin Glaciation. When the water receded, it left behind deposits of clay and sand. Wet areas of the preserve are covered with clay, while drier areas are covered with sand. The ancient beach ridge formed by this lake extends from southeast to northeast across the preserve, creating a variety of topography.

Gensburg-Markham Prairie provides a unique example of sandy loam prairie, combining characteristics of black silt prairie and true sand prairie. The preserve includes rare plant species such as grape fern, primrose, narrow-leaved dewberry, and pink orchid. Other significant sand prairie plants found here include rootwort, yellow-eye, rifleweed, and blueberry. The dominant grasses are big bluestem, Indian grass, and prairie seed drop. Typical black soil plant species found here include drooping onion, wild indigo, prairie coreopsis, and prairie root.

Of particular note are nesting birds such as short-billed marsh wren, bobolink, swamp sparrow, and Henslow’s sparrow.