During the Late Pleistocene, the Laurentide ice sheet reached from the Rocky Mountains eastward through the Great Lakes, into New England, covering nearly all of Canada east of the Rocky Mountains.
The Columbia begins its 1,243-mile (2,000 km) journey in the southern Rocky Mountain Trench in British Columbia (BC).
Columbia River Basalt Group map shows main regions of basalt exposure in Washington, Oregon, Idaho and Nevada, USA. (usgs.gov)
