The rocks of the Blue Ridge physiographic province bear witness
to the power of large scale volcanism, metamorphism, and
erosion. They are the oldest rocks in the National Capital
Region and form the “basement” for all the other rocks. The Blue
Ridge province was once part of an ancient supercontinent that
existed 1 billion years ago in the Proterozoic Eon.
Approximately 600 million years ago, rifting began to pull this
supercontinent apart. The rift created a basin for a new ocean
which existed long before the Atlantic, the Iapetus Ocean. It
also left behind igneous rocks, which were formed when magma
rising from deep within the Earth erupted on the surface. This
is now the most common type of rock of the Blue Ridge province.
As the Iapetus Ocean grew larger and the land masses spread
farther apart, river and marine sediments were deposited on top
of the igneous rocks. Eventually the Iapetus Ocean began to
close, and three different mountain-building episodes occurred
when land masses collided into the ancie