The rest of the island is composed of a great variety of Paleozoic rocks of sedimentary, igneous and metamorphic origin. Along the west coast lie the Long Range Mountains, which are formed by an elongated block of the Earth's crust (a horst) which rises to about above sea level. This part of the island was once part of the eastern margin of continental North America. The island's highest points, the Lewis Hills and Gros Morne, are located within this mountain range. To the east is a depression or graben about wide, which is occupied by Deer Lake and Grand Lake. The main plateau of the central part of the island, which was once the sea bottom of the ancient Iapetus Ocean, has been heavily eroded by water and ice. Steep, solitary rock knobs, called "tolts" in Newfoundland (elsewhere known as inselbergs or monadnocks), which jut or more above the generally flat terrain are the remnants of a former higher landscape level. Glaciers which helped shape these tolts left other evidence around Newfoundland. Large blocks of stone called glacial erratics have been left scattered across much of the landscape. The long narrow lakes of the west coast, notably those in Gros Morne National Park resulted from glacial erosion. The lack of good soil on most parts of the island is a result of the scouring effect of glaciers during the most recent ice age. Newfoundland's nickname, "The Rock", is partially a result of the ice ages.