The Puget lobe, the southwesternmost extension of the Cordilleran ice sheet, last advanced into the Puget Lowland of western Washington at about 15 ka. The advancing ice sheet deposited voluminous sediment on a prograding, proglacial outwash plain that extended from the Olympic Mountains to the Cascade Range, herein recognized as the "great Lowland fill." Subsequent overrunning by the ice sheet excavated deep linear troughs now occupied by the large lakes and marine waters of Puget Sound. Excavation of these troughs and valleys of the Puget Lowland required the net transport of about 1000 km3 of sediment, almost entirely during ice occupation and primarily by subglacial water. These landforms of glaciofluvial deposition and erosion define the modern landscape here, emphasizing the importance of these processes in the region's geomorphology.

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