The rocks in Harriman and Bear Mountain State Parks record a long and complex history that began 1.3 billion years ago and continues today. Most of the rocks were deposited as sediments and volcanics about 1.3 billion years ago in a setting similar to Japan today. The composition of the volcanic rocks tells us that the setting was an island arc above a subduction zone. About 1.0 to 1.1 billion years ago, a huge mountain building event called the Grenville Orogeny resulted from a continental collision like that which built the Himalayas. This event turned all of the rocks into the gneisses that we see today. The rocks were buried to about 25-30 km depth during this event and heated to over 700°C. A second phase of the Grenville orogeny was the formation of large strike-slip faults similar to the San Andreas Fault, CA of today. Late in the history of this faulting, the faults became dilational and metamorphic fluids deposited magnetite (iron ore) and related minerals into veins of several miles in length. These veins were deposited 923 million years ago. The magnetite was mined for iron for two centuries in early American history. The Grenville Orogeny was one of several that built the supercontinent Rodinia. There was a period of tectonic quiescence for over 200 million years until Rodinia broke-up in a worldwide rifting event.