It rises above Nockamixon State Park, in the Delaware River drainage of southeastern Pennsylvania.

[7] Haycock Mountain was formed 200 million years ago by an intrusion of magma into local shale and argillite within the Newark Basin.

Millions of years of weathering then stripped away overlying layers of shale and argillite to expose the durable diabase to the surface, creating Haycock mountain as it appears today.

[8] In addition to diabase, Haycock Mountain features a large area underlain by hornfels, or local sedimentary rock that has been baked by the heat of the magma intrusion.

The baked rock is most prominent within a few hundred feet of the intrusion, appearing as a dark gray hornfels.