There are thousands of glaciers in Alaska but only few have been named. The Columbia Glacier near Valdez in Prince William Sound has retreated in the last 25 years. Its calved icebergs partially caused the Exxon Valdez oil spill, when the tanker changed course to avoid the ice tips. The Valdez Glacier is in the same area, and though it does not calve, has also retreated significantly. "A 2005 aerial survey of Alaskan coastal glaciers identified more than a dozen glaciers, many former tidewater and calving glaciers, including Grand Plateau, Alsek, Bear, and Excelsior Glaciers that are rapidly retreating. Of 2,000 glaciers observed, 99% are retreating." Icy Bay in Alaska is fed by three large glaciers—Guyot, Yahtse, and Tyndall Glaciers—all of which have experienced a loss in length and thickness and, consequently, a loss in area. Tyndall Glacier became separated from the retreating Guyot Glacier in the 1960s and has retreated since, averaging more than per year.