After receiving his Ph.D. in 1959, Cox joined the United States Geological Survey in Menlo Park, California. There he collaborated with another geophysicist, Richard Doell, on rock magnetism. The two were particularly interested in geomagnetic reversals. At the time, very little was known about the timing of reversals. The rock specimens they collected were too young (a few millions of years) to date accurately until the potassium-argon dating method was developed. Cox and Doell arranged for the USGS to hire Brent Dalrymple, a graduate from Berkeley with expertise in this method. The three succeeded in creating the first geomagnetic polarity time scale. This work made possible the first test, by Frederick Vine and Drummond Matthews, of the seafloor spreading hypothesis.