The ROCK's bedrock

The ROCK is perched on a rare topographic location where marble and other metamorphic stones were formed millions of years ago. A rock garden at the site is built over the marbled ledge of the ridge, where quartz , schist, shale and other rocks are found.

To the west of the ridge is the Harlem Valley, underlain by the Stockbridge marble. The same rocks are found to the east in the valley on the other side of the ridge in Connecticut. These were deposited as limestones in shallow waters at the edge of a warm sea, about 500 million years ago, and have been subsequently turned into marble by heating and high pressure during a continental collision. In this narrow geological area, all the major valleys are underlain by the Stockbridge marble, which is more susceptible to erosion due to our slightly acidic rainfall.

The ridge itself is made up of the Walloomsac Formation. This was originally deposited as black shales in an anoxic basin off the coast from the Stockbridge limestones. When it was heated and compressed in the same continental collision that turned the limestone into marble, mica grains grew perpendicular to the main compression, giving the rock a shiny color and an oriented cleavage or schistosity. The temperature and pressure were high enough that the minerals garnet and staurolite could grow in beds of the right composition.