Lyon Mountain, at the northeast end of the park, was a large iron ore production center for nearly a century. Mining took place underground, to a depth of 3,500 feet, in some of the deepest mines in the northeast. The mines operated from the late 1800s to 1967, mostly developed by the Chateaugay Ore and Iron Company, later acquired by Republic Steel. Its ore was used in the construction of the Brooklyn Bridge, the George Washington Bridge, and the Golden Gate Bridge. The company town of Lyon Mountain had a population of 3,000 at its peak (after World War II) and dropped to around 300 after the mine closed. The empty high school was turned into a prison, but in 2011, that closed too. The principal mine shaft and processing site is on the hill behind town, where a few abandoned buildings remain, fenced off and overgrown. A mile from town, a hoist tower over another mine shaft is now part of an RV park, and is used as a cell tower. Ore sand and slag can be found in piles scattered around the town, sometimes overgrown, sometimes not.