#glacial erratic

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The real story behind the world's 'wandering rocks'

If you've ever come across a rock that looks strangely out of place, odds are it's a glacial erratic. They've become draws for travelers and the focus of local legends—but these rocks have stories of their own to tell.

Central Park, Manhattan (No. 3)

There are four different types of bedrock in Manhattan. In Central Park, Manhattan schist and Hartland schist, which are both metamorphosed sedimentary rock, are exposed in various outcroppings. The other two types, Fordham gneiss (an older deeper layer) and Inwood marble (metamorphosed limestone which overlays the gneiss), do not surface in the park. Fordham gneiss, which consists of metamorphosed igneous rocks, was formed a billion years ago, during the Grenville orogeny that occurred during the creation of an ancient super-continent. Manhattan schist and Hartland schist were formed in the Iapetus Ocean during the Taconic orogeny in the Paleozoic era, about 450 million years ago, when the tectonic plates began to merge to form the supercontinent Pangaea. Cameron's Line, a fault zone that traverses Central Park on an east–west axis, divides the outcroppings of Hartland schist to the south and Manhattan schist to the north.

Various glaciers have covered the area of Central Park in the past, with the most recent being the Wisconsin glacier which receded about 12,000 years ago. Evidence of past glaciers can be seen throughout the park in the form of glacial erratics (large boulders dropped by the receding glacier) and north–south glacial striations visible on stone outcroppings. Alignments of glacial erratics, called "boulder trains", are present throughout Central Park. The most notable of these outcroppings is Rat Rock (also known as Umpire Rock), a circular outcropping at the southwestern corner of the park. It measures 55 feet (17 m) wide and 15 feet (4.6 m) tall with different east, west, and north faces. Boulderers sometimes congregate there. A single glacial pothole with yellow clay is near the southwest corner of the park.

The underground geology of Central Park was altered by the construction of several subway lines underneath it, and by the New York City Water Tunnel No. 3 approximately 700 feet (210 m) underground. Excavations for the project have uncovered pegmatite, feldspar, quartz, biotite, and several metals.