South Mountain Reservation is a large wooded county park in central Essex County, New Jersey, occupying rugged terrain along the Rahway River between the first and second Watchung ridges. Assembled through purchases beginning in 1895 and shaped in part by Frederick Law Olmsted and Olmsted Brothers, it was intentionally preserved in a near-wild state, with forests, streams, ponds, waterfalls, overlooks, and recreation areas. The Orange Reservoir within it was later adapted from water supply use to recreation. The reservation also preserves Washington Rock, where a Revolutionary War beacon station helped warn local militia on June 23, 1780, of a British advance toward Hobart Gap.