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 savannahs cover the central portion of the state. The Ozark and Ouachita Mountains rise from west to east over the state's eastern third, gradually increasing in elevation in an eastward direction.