Rutan Hill
Rutan Hill, also known as Volcanic Hill, is a small hill in Wantage Township, Sussex County, New Jersey. It sits on private land, about 2.5 miles south-southwest of Colesville. The hill rises roughly 270 feet above the nearby creek valley, reaching just over 1,020 feet in elevation.
Geologists see Rutan Hill as the surface expression of a diatreme, which is part of the Beemerville Alkaline Complex, a very old volcanic-related intrusion from the Late Ordovician period. The Beemerville Complex includes two large nepheline syenite rock bodies and several smaller dikes and diatremes in the area. Rutan Hill is the largest lamprophyric diatreme here and contains a small plug-like body of nepheline syenite about 30 meters across. It is filled with many pieces of surrounding rocks (xenoliths) such as shale, graywacke, dolomite, and limestone, plus chunks of nepheline syenite and carbonatite. The surrounding matrix is very fine, with crystals like biotite, diopside, aegerine-augite, orthoclase, magnetite, apatite, and nepheline.
In the bigger picture, Rutan Hill marks the surface of a much larger alkaline intrusion beneath Beemerville and is part of the Cortlandt-Beemerville magmatic belt, which runs across northern New Jersey and southeastern New York. The Beemerville rocks formed about 420 million years ago, with other dating showing later cooling. The dikes in the area cut through older Martinsburg Formation rocks, indicating the intrusion came after those rocks formed. The diatreme probably represents the throat of an ancient, extinct volcano; erosion likely removed the volcanic cone and any surface deposits, so no volcanic features are visible today.
This volcanic activity happened toward the end of the Taconic Orogeny in the Late Ordovician, in a tectonic setting tied to the closing of the Iapetus Ocean as landmasses collided near the eastern edge of North America.
This page was last edited on 2 February 2026, at 15:34 (CET).