Detailed thermal demagnetizations of samples from a 15 million year old lava flow from the Sheep Creek Range in Nevada show that the ancient geomagnetic field direction changed by over 80? during the time it took the flow interior to cool from 680?C to 375?C. The samples used in these experiments were approximately 1-inch in diameter and 1-inch in length and were oriented in the field using a sun compass. The lava flow (referred to as E1) is 5 meters thick and the samples were drilled every half-meter through its thickness. The direction of the magnetic field immediately prior to the eruption of E1, pointing steeply upward and NW is recorded by the underlying flow W7. The field direction following the eruption of E1, shallow and SE, is defined by a series of overlying lava flows all with the same paleomagnetic direction. All the lava flows formed during a polarity reversal of the geomagnetic field. The sample from nearest the flow base contains a single characteristic magnetization identical to that of the underlying flow. Seven samples from the interior of the flow contain composite magnetizations. The high temperature component (acquired first during cooling) is similar to the direction recorded by the flow base. The lower temperature components (acquired later as cooling progressed) drift closer to the direction of the overlying flow. The sample from the flow top was apparently completely remagnetized by the heat from the overlying flow. These paleomagnetic data are very similar to those gathered from Steens Mountain, Oregon. Steens Mountain data are interpreted as evidence for very rapid field change of 3? per day. If a similar rate is recorded by the Sheep Creek samples, then cooling from 680?C to 375?C took approximately 27 days.