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 wide and 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 underground. Excavations for the project have uncovered pegmatite, feldspar, quartz, biotite, and several metals. Wooded areas and lawns There are three wooded areas in Central Park: North Woods, the Ramble, and Hallett Nature Sanctuary. North Woods, the largest of the woodlands, is at the northwestern corner of Central Park. It covers about adjacent to North Meadow. The name sometimes applies to other attractions in the park's northern end; these adjacent features plus the area of North Woods can be . North Woods contains the Ravine, a forest with deciduous trees on its northwestern slope, and the Loch, a small stream that winds diagonally through North Woods. The Ramble is in the southern third of the park next to the Lake. Covering , it contains a series of winding