Given the following context:  Oklahoma is between the Great Plains and the Ozark Plateau in the Gulf of Mexico watershed, generally sloping from the high plains of its western boundary to the low wetlands of its southeastern boundary. Its highest and lowest points follow this trend, with its highest peak, Black Mesa, at 4,973 feet (1,516 m) above sea level, situated near its far northwest corner in the Oklahoma Panhandle. The state's lowest point is on the Little River near its far southeastern boundary near the town of Idabel, which dips to 289 feet (88 m) above sea level. Among the most geographically diverse states, Oklahoma is one of four to harbor more than 10 distinct ecological regions, with 11 in its borders—more per square mile than in any other state. Its western and eastern halves, however, are marked by extreme differences in geographical diversity: Eastern Oklahoma touches eight ecological regions and its western half contains three. Although having fewer ecological regions Western Oklahoma contains many rare, relic species. Oklahoma has four primary mountain ranges: the Ouachita Mountains, the Arbuckle Mountains, the Wichita Mountains, and the Ozark Mountains. Contained within the U.S. Interior Highlands region, the Ozark and Ouachita Mountains are the only major mountainous region between the Rocky Mountains and the Appalachians. A portion of the Flint Hills stretches into north-central Oklahoma, and near the state's eastern border, The Oklahoma Tourism & Recreation Department regards Cavanal Hill as the world's tallest hill; at 1,999 feet (609 m), it fails their definition of a mountain by one foot.The semi-arid high plains in the state's northwestern corner harbor few natural forests; the region has a rolling to flat landscape with intermittent canyons and mesa ranges like the Glass Mountains. Partial plains interrupted by small, sky island mountain ranges like the Antelope Hills and the Wichita Mountains dot southwestern Oklahoma; transitional prairie and oak savannas cover the central portion of the state. The Ozark and...  answer the following question:  What fails the definition of a mountain by one foot?
Answer:
Cavanal Hill