Photographs and on-the-ground surface evidence suggest that a ‘young’ fault runs the length of strategic O'Neill Forebay Dam in Merced County, central California. Darrell Herd, a U.S. Geological Survey geologist in Menlo Park, California, is the author of a new report on the area. He discovered the fault during recent investigations of faulting in the San Francisco Bay region. The fault, now called the O'Neill fault, is part of a larger zone of faults which Herd recently discovered on the east side of the Diablo Range, a part of the Coast Ranges. Herd believes that the fault runs from near Tracy, California, south to near Los Banos, California, and that it is geologically young because some evidence indicates activity within the last 10,000 years.