Deglaciation of Chilliwack River valley, British Columbia

Quaternary sediments and landforms in Chilliwack River valley, southwestern British Columbia, provide a detailed record of déglaciation of this area between 12 000 and 11 000 years BP. Stratigraphic, sedimentological, and radiocarbon data show that a large glacier in eastern Fraser Lowland (part of the Cordilleran Ice Sheet) blocked the mouth of Chilliwack valley at a time when the middle reaches of the valley were ice free. A lake existed between the ice dam in the lower part of the valley and a delta – sandur complex, west of Chilliwack Lake, in the upper part of the valley. Two relatively minor advances of the Fraser Lowland ice lobe into lower Chilliwack valley occurred about 11 500 and 11 200 years BP. These were separated by a brief period of recession during which tephra was deposited and a coniferous forest and soil developed on freshly deglaciated terrain. Shortly after 11 200 years BP, a glacial lake formed in Cultus Lake basin; two sets of terraces in lower Chilliwack valley are graded to different levels of this lake. The glacier dam at the mouth of Chilliwack valley disappeared about 11 000 years BP, and déglaciation of Fraser Lowland was complete less than 100 years later.