Thrust faulting from subduction in the Aluetian trench over the past 20million years has gradually uplifted the Chugach Range in the Prince WilliamSound area. This range at first was most probably an uplifted plateau. About 20,000 years ago, the Earth's climate cooled and the last of the greatPleistocene ice ages occurred. Glaciers formed in the streambeds of thecoastal plateau and carved deep valleys. Glaciers form because warm, low pressure systemssweeping in off the Pacific Ocean in the winter encounter the mountains,rise, cool and deposit their excess moisture.
More snow falls in the longwinter than melts during the short summer.
The thick, accumulating snowlayers compress into ice which gradually flows down to the sea as glaciers. Columbia Glacier Tidewater glaciers exhibit cyclical behavior. Columbia Glacier, the Sound'slargest tidal glacier, entered the drastic retreat phase of its cycle in1984 discharging huge quantities of icebergs that totally clogged the areabehind its former moraine and often filled the outer bay with ice. In Novemberof 1995, the glacier's retreat slowed and both the outer and inner baysbecame ice-free. It is too soon to know whether this is a temporary pauseor whether the glacier has stopped retreating.