Complex sentence: The cliffs along the Delaware River, known as Nockamixon Cliffs, appear to rise from the land, but are in fact formations of very hard stone that have eroded at a much slower pace than the surrounding land.
Split into simpler sentences: The cliffs along the Delaware River, known as Nockamixon Cliffs, appear to rise from the land. They are in fact formations of very hard stone that have eroded at a much slower pace than the surrounding land.