The glider was launched by aerotow and released about 5 miles from the departure airport. After climbing to about 10,400 ft msl, the pilot flew the glider towards an adjacent peak, however while enroute the glider encountered sink and descended. Rather than fly back towards the sinking air, the pilot began to track towards another peak in anticipation of finding lifting air, in an area where he had been successful in the past. However, the glider continued to descend so he turned back towards the departure airport. The headwinds were significant, and by the time the glider had sunk to 1,000 ft above ground level, the airport was no longer in sight, so he elected to perform an off-field landing on a dirt road. During the landing roll the left wing struck a raised rock, which resulted in substantial damage to the left wing and fuselage. The pilot reported no preimpact mechanical malfunctions or anomalies that would have precluded normal operation.