The Marlboro Mountains are a series of hills that extend from Newburgh to Kingston. Like the Catskills, the Marlboros are composed of ancient sedimentary rock but they much, much older.
The Marlboros were formed during the Ordovician period which began almost half a billion years ago.
The Ordovician spans 42 million years from the end of the Cambrian Period (485 million years ago) to the start of the Silurian Period (443 million years ago.) The Ordovician period was marked by the emergence of the first jawed fish, and the first land organisms in the form of simple plants.
The Devonian period (during which The Catskills formed) followed the Silurian — so the order is Ordovician › Silurian › Devonian.