The Round Valley Reservoir in Clinton Township in Hunterdon County, New Jersey, United States, was formed in 1960 when the New Jersey Water Authority constructed two large dams and flooded a large valley. The reservoir is named after the naturally formed 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.