The end of the Triassic saw large-scale rifting during the break-up of Pangaea. What is now eastern North America began to separate from what is now north-western Africa, creating the young Atlantic Ocean. Magma came from decompression melting, and some was pushed into the pre-existing sandstones and arkoses. The magma solidified and, after millions of years, the overlying rocks got uplifted and eroded, exposing the Palisades Sill as we see it.