The Round Valley Reservoir in the U.S. state of New Jersey was formed in 1960 when the New Jersey Water Authority constructed two large dams and flooded a large ravine in the state's Hunterdon County. The Reservoir is named after the naturally forming circular valley surrounded by Cushetunk Mountain. The deep Valley was caused by erosion of the soft sedimentary rock. The surrounding ridges of Cushetunk Mountain endure because they were underlaid with dense and durable volcanic rock diabase that cooled slowly under the surface of the earth. Round Valley is one of the few parks that offers wilderness camping. The campsites on the eastern side of the reservoir are accessible only by hiking or boating. The campers' parking lot is three miles away from the nearest site, reachable by a steep and rugged trail. Cross-country skiers and sledding enthusiasts congregate at Round Valley in the winter months.