The fastest-moving major glacier is the Columbia glacier, between Anchorage and Valdez, in Alaska, USA. In 1999 it was measured to be flowing at an average rate of 35 m (115 ft) per day. The glacier has almost doubled its previously recorded speed of 20 m (65 ft) per day, possibly due to global warming. Glaciers are large moving masses if ice formed in (usually mountainous) regions where the rate of snowfall is greater than the rate of snow melting. The snow piles up, is compressed into ice by the weight of more snow above it, and begins to slowly flow downhill under the force if its own weight. When a flowing glacier meets the sea, chunks of it can break off to form icebergs.