The Columbia River Basalt consists of dozens of seemingly identical flows of basalt covering thousands of square miles of Oregon, Washington, and Idaho. For years, detailed mapping of the units relied almost entirely on subtle petrographic distinctions, the presence or absence of interbeds, and actual walking along contacts in the field. Eventually two divisions were recognized: Yakima Basalt and Picture Gorge Basalt. Further detailed work in southeastern Washington revealed distinctive and laterally continuous flows within the Yakima Basalt. In recent years, geochemical and geophysical techniques have been used to supplement more traditional geologic approaches, allowing geologists to map with much greater precision. The paleomagnetic technique described in this article illustrates how new technology is assisting the geologist in his work.