Many beaches in southern Alaska, and other beaches at high latitudes in the Northern Hemisphere, consist of gravel, small rocks, and boulders. These materials come from coarse glacial till—mixtures of clay, silt, sand, gravel, and rocks that were carried to a location by ancient glaciers, remaining there when the glaciers melted. The till has usually been reworked by wave action, with the lighter material (clay, silt, and sand) washed away and the heavier gravel and rocks sorted by size and deposited in different areas along the shoreline. Such is the case in Columbia Bay, a region within Alaska’s Prince William Sound. Many of the beaches in this area have old tidal lines visible above the present ones, the result of a huge earthquake in 1964 that raised the land by 8 ft (2.4 m).