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.