The Interior Low Plateau ecoregion occupies portions of six states in the Midwest and South East regions of the United States, Illinois, Indiana, Ohio, Kentucky, Tennessee and Alabama. Compared with other ecoregions, it is of average size, covering 47,769,500 acres; (74,639 square miles). Surficial geology is of two primary types: on the northern edges on the ecoregion in Ohio and Indiana, and over much of Illinois, Illinoian till dominates the landscape. To the south, the bulk of the ecoregion is characterized by unglaciated limestone and related geology. Elevation ranges from a high in Illinois of 1,060 at Williams Hill, to a low of 325 ft. along the Ohio River as it leaves the ecoregion in Illinois and Kentucky. Most of the ecoregion lies between 500 and 850 ft. in elevation. Much of the unglaciated portion of the ecoregion is characterized by rolling limestone plains punctuated with regions of fairly rugged topography, and 250 feet of relative topographic relief is common in many areas (maximum relative relief is over 500 ft).

Topography is mostly hilly and rolling, with areas of swampy alluvial valleys, deeply entrenched rivers and streams, and expansive karst plains. Several large rivers traverse the ecoregion, including the Ohio, Tennessee, Cumberland Kentucky and Licking Rivers. Originally the unglaciated portion of the ecoregion was dominated by expansive forest systems, although extensive prairies and barrens and oak savanna dominated portions of karst plains. Interspersed throughout the unglaciated ecoregion were caves, glades, and swamps, which today remain as biologically diverse conservation targets. The Illinoinan Tillplain was characterized by rolling topography that was dominated by a prairie forest ecotone in Illinois and forested in the eastern portions.