Lake Washington near Seattle occupies a deep narrow trough sculptured by the Vashon ice sheet, the last continental glacier to invade the Seattle area. Extending along the center of the trough is a broad ridge that stands 5 to 30 feet above narrow valleys on either side. Thus, the trough in cross-section is W-shaped rather than U-shaped, as are most glacial valleys. The sediments in the trough consist of blue clay, locally more than 100 feet thick, overlain by limnic peat or gyttja 5 to 55 feet thick. The blue clay consists of rock flour of meltwater origin and the limnic peat consists of planktonic organisms that began to accumulate following the meltwater stage....