Light-colored granite, about 50 to 70 million years old (Eocene Epoch to Cretaceous Period) has been sculpted by glaciers that gouged deep U-shaped troughs throughout the monument. Many of the glacial valleys are filled with sea water and are called "canals", but they are not man-made in any way; the walls of these valleys are near-vertical and often rise 2,000 to 3,000 feet (600 to 900 m) above sea level, and drop 1,000 feet (300 m) below it.