As the ice that covered this land slowly melted, it dropped great accumulations of gravel and rocks. Boulders transported and deposited by glaciers are called erratics, and they are rounded and noticeably different in composition from local bedrock. Glaciers carry huge loads of rock, which come to rest where the melting ice drops them; the large boulder perched on the South Bubble is a striking example. About 20,000 years ago, before the ice sheet came, the rugged Acadian highland was a granite ridge running east-west. By 17,000 years ago, huge tongues of ice had invaded the valleys on the north slopes, cut through the passes, and engulfed the mountains on their way to the sea. At its maximum 15,000 years ago, the ice covered Acadia with a sheet over 8,000 feet thick and extended about 200 miles seaward to the edge of the continental shelf. By 13,500 years ago, climatic warming caused the ice to melt back to the north, and ocean waters rose, flooding the foothills of the mountains. From 10,000 years ago to the present, the ice changed the shape of Acadia by cutting deep U-shaped valleys running north-south and rounding and polishing the mountaintops.