that occurred as a result of igneous intrusions. Sourland Mountain was formed 200 million years ago by a plutonic
intrusion of a hard
igneous rock,
diabase, into the
Brunswick Shale. The intrusion occurred as tectonic forces created the
Newark Basin, a prehistoric rift valley active near the rift which ultimately spread North America and Africa apart. The parallel ridges of the mountain were probably formed by magma seeping up through linear faults created during the rifting process During the early
Jurassic the rift which formed the Newark Basin failed, and the intrusion which would eventually become Sourland Mountain ceased growing. Initially, the vast majority of volcanic material remained within the crust. However, the diabase and related
hornfels which form the backbone of the mountain are a type of highly erosion resistant stone known as
trap rock. Consequently, millions of years of erosion lowered the more easily worked shale surrounding the intrusion, turning it into the elevated ridges which currently comprise Sourland Mountain. The
Palisades Sill, in northeastern New Jersey and southeastern New York, was not only formed under similar circumstances to Sourland Mountain, it is at least partly contiguous with it. The ridge forming Mount Rose, Rocky Hill, and Ten Mile Run Mountain, is a continuation of the Palisades which connects to Pennington Mountain. South of Pennington Mountain the Palisades sill and Sourland Mountain's eastern ridge are essentially the same feature. Like the Palisades, Sourland Mountain exhibits volcanic dikes which extend away from the main ridge. One such dike extending northwest from Sourland Mountain has been determined to be responsible for the creation of Prospect Hill, an extrusive basalt peak near Flemington, New Jersey. Prospect Hill marks the only known location where Sourland Mountain's magma actually erupted to the surface. In this respect, it is similar to
Snake Hill, which it most likely a volcanic plug linked to the Palisades. 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. ==Wildlife==