200 million years ago, magma intruded into the Newark Basin, then an active rift basin associated with the breakup of the supercontinent Pangaea. The magma was initially contained within the sedimentary strata of the basin, forming large intrusions like the Palisades Sill, but it ultimately broke out to the surface through large, episodic eruptions. The Watchung Mountains were originally formed from these eruptions, consisting of three separate flood basalts that may have filled nearly the entire Newark Basin. Each time the basin filled with basalt, which cooled into blocky trap rock, a period of limited volcanic activity followed, allowing sediment to be deposited on top of the previously erupted layer of basalt. In this way, the Newark Basin became layered with alternating strata of Watchung basalt and Jurassic sedimentary rock.