premise: Rockcastle County is a county located in the U.S. state of Kentucky. As of the 2010 census, the population was 17,056. Its county seat is Mt. Vernon. The county founded in 1810 and named for the Rockcastle River which runs through it. The river, in turn, is named for its majestic rock cliffs.
hypothesis: rockcastle was named after its discoverer
reason: rockcastle was named for the Rockcastle River which runs through it, not after its discoverer