Approximately 17,000 years ago the terminal moraine of the receding Wisconsin Glacier that formed Long Island, known as the Harbor Hill Moraine, established a string of hills and kettles in the northern part of the park and a lower lying outwash plain in the southern part. Mount Prospect (or Prospect Hill), near the intersection of Flatbush Avenue and Eastern Parkway, is one of the tallest hills in Brooklyn, rising 200 feet (61 m) above sea level. It is the highest among a string of hills that extends into the park, including Sullivan, Breeze, and Lookout hills. The area was originally forested, but became open pasture after two centuries of European colonization. Significant stands of trees remained only in the peat bogs centered south of Ninth and Flatbush Avenues, as well as in a large bog north of Ninth Street, and contained chestnut, white poplar, and oak. Some of these stands were preserved in the modern-day Prospect Park Ravine and nicknamed "The Last Forest of Brooklyn".