The pilot indicated that the experimental amateur-built biplane lost engine power during cruise about 20 minutes into a cross-country flight. The terrain where the airplane sustained substantial damage to its lower left spar during an off field landing consisted of rolling hills. He reported that he departed with approximately 10 to 11 gallons of 100 low lead aviation gasoline. The pilot’s accident report stated the airplane remained intact following the accident. An on-scene examination of the wreckage revealed no pre-impact anomalies and the fuel system was intact. The indicated on-scene fuel level was in a red colored range on the fuel sight gauge.