“ Its surface is unbroken by marked elevations, but the monotony is relieved by the broad undulations and by the channels of the streams tributary to the Ohio, Mississippi, and Missouri, which have been worn down in places so as to expose vertical walls or bluffs 100 feet or more in height. The elevation above sea-level ranges from 300 to 1500 feet. The prairies are underlaid by Paleozoic sandstones and limestones in nearly horizontal position ”