Retreat at an ancient coral reef
...Imagine a shallow sea on the continental shelf of eastern North America, back before anything lived on land. With various, very different kinds of marine life living on the seafloor, and swimming through the seas; Like the long-extinct trilobites.
Chuck Ver Straeten
NY State Museum/Geological Survey
At the ridge, you will be standing at an ancient beach. The retreat is perched on a rare topographic location where marble and other metamorphic stones were formed about 500 millions of years ago. The rock garden at the site showcases the marbled ledge of a Cambrian ridge, where also lays quartz, schist, shale, and other rocks. The bedrock is metamorphosed "carbonate" rocks (limestone and dolostone), resulting from transformations caused by elevated pressures and heat.
Jeff Walker, Professor of Geology at Vassar College, explains, "these rocks originally formed as limestone and dolostone in shallow seas along the eastern margin of North America between about 520 to 470 million years ago, during the geological Cambrian and Ordovician periods. They were later buried deep below the ancient Appalachian Mountains during continent-to-continent type collisions, much as India collided with southern Asia over the last 30 million years, forming the Himalaya Mountains. It was while they were buried deep under the Appalachians when it was a young, tall, and rugged mountain belt, that the rocks were metamorphosed, miles below the mountaintops. Erosion over the last hundreds of millions of years led to their exposure today, along the ridge."
Jeff Walker, Professor at Vassar Collegue, states, "to o the west of the ridge is the Harlem Valley, underlain by the Stockbridge marble. In this narrow geological band, all the major valleys are underlain by this type of marble, which is more susceptible to erosion due to the slightly acidic rainfall in the area. 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 continental collision."
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.
Recent excavation in the north west area of the ridge, where the beach stands, led to the discovery of fossils, likely eels and possibly trilobites. An effort to uncover further fossils or vestiges of prior eras is now underway. We welcome volunteers who want to join us in this archeological exploration; you may contact us at eco@therockattheridge telling us a bit about you and your interest in this project. Thank you.
Thank you to New York's rock(ing) experts, geologists Charles Ver Straeten, Marian Lupulescu and Jeff Walker.