The coastline of Acadia in Maine is one of the most spectacular in the northeastern US. It now forms the Acadia National Park, most of which is found within a single large island, Mount Desert Island, and some smaller associated islands. Sea-level rise since the last ice age has separated these islands from each other and from the mainland. The mountains that make up the basis of this coastline began to form 500 million years ago from seafloor sediments. Magma (molten rock) rising up from Earth’s interior intruded into and consumed these sedimentary rocks, producing a mass of granite that was gradually eroded to form a ridge. About 2–3 million years ago, a huge ice sheet started to blanket the area, depressing the land and sculpting out a series of mountains separated by U-shaped valleys. Since the ice sheet receded, the land has gradually rebounded upward, but global sea-level rise has caused the Atlantic to overtake the rebound at a rate of 2 in (5 cm) per century. Today, waves and tidal currents are major agents of change at Acadia, gradually eroding the cliffs and depositing rock particles mixed with shell fragments at coves around the coastline.