THE PILOT REPORTED THAT WHEN THE ACCIDENT OCCURRED, HE WAS SOARING HIS "MOTOR-GLIDER" IN UPDRAFTS OF "THERMAL, SLOPE-WIND, AND WAVE" ORIGIN. HE REPORTED THAT THERMALS AND A WEAK SOUTHWEST WIND WERE PRODUCING LIFT ALONG RIDGES WITH WALLS THAT WERE FACING WEST, SOUTHWEST, AND SOUTH. REPORTEDLY, HE WAS FLYING IN THESE AREAS, IN AND OUT OF CANYONS, WITH THE ENGINE OPERATING AT A FAST IDLE TO REDUCE DRAG. WHILE FLYING WESTWARD, HE ENCOUNTERED AN AREA OF LIFT WHERE HE MANEUVERED AND CLIMBED FROM 2900 TO 3100 FT. FROM THERE, HE STARTED TO FOLLOW A WEST FACING WALL OF A NORTH CONNECTING RIDGE WHEN HE ENCOUNTERED A SUDDEN, VERY STRONG, DOWNDRAFT. HE APPLIED FULL THROTTLE AND TURNED LEFT TO FOLLOW A CANYON THAT DROPPED AWAY TO THE WEST, BUT THE AIRCRAFT STRUCK A ROCKY OUTCROPPING AT AN ELEVATION OF APPROXIMATELY 3000 FT MSL.