Surrounded by the knobs region on the south, west, and east and extending as far west as Louisville is the bluegrass country, the heart and trademark of the state. To the south and west lie the rolling plains and rocky hillsides of the Pennyroyal, a region that takes its name from a species of mint that grows abundantly in the area. There, underground streams have washed through limestone to form miles of subterranean passages, some of the notable ones being in Mammoth Cave National Park. Northwest Kentucky is generally rough, rolling terrain, with scattered but important coal deposits. Consisting of floodplains and rolling uplands, it is among the largest migratory bird flyways in the United States.
Rivers are an important feature of Kentucky geography.
The Ohio River forms the entire northern boundary of the state, flowing generally SW below Covington, until it joins the Mississippi River W of Paducah. In the east, the Big Sandy River and its tributary, the Tug Fork, form the boundary with West Virginia. Many rapid creeks in the Cumberland Mountains feed the Kentucky, the Cumberland, and the Licking rivers, which, together with the Tennessee and the Ohio, are the chief rivers of the state. The Kentucky Dam on the Tennessee River near Paducah, is a major part of the Tennessee Valley Authority system. Kentucky's climate is generally mild, with few extremes of heat and cold.