Mount Monadnock, solitary mass of rock (3,165 feet [965 metres]) in Monadnock State Park, southeast of Keene, southwestern New Hampshire, U.S. It is a classic example of, and gave its name to, the geologic feature called a monadnock. Mount Monadnock was celebrated by Ralph Waldo Emerson in the long poem Monadnoc (1846) and was a favorite hiking destination of Henry David Thoreau.