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.[1][6][7] Alignments of glacial erratics, called "boulder trains", are present throughout Central Park.[8] The most notable of these outcroppings is Rat Rock (also known as Umpire Rock), a circular outcropping at the southwestern corner of the park.[6][9] It measures 55 feet (17 m) wide and 15 feet (4.6 m) tall with different east, west, and north faces.[9][10] Boulderers sometimes congregate there.[10] A single glacial pothole with yellow clay is near the southwest corner of the park.[11][12]