One of the key differences between Sourland Mountain and the other volcanic mountains of the Newark Basin, including the Watchungs and the Palisades, is that it has not been touched by glaciers. This has left the mountain with a mantle of Triassic argillite, which covers much of the underlying trap rock and comprises the bulk of Pheasant Hill. The impermeability of argillite is primarily to blame for Sourland Mountain's extremely poor aquifer.