The student pilot and his instructor were returning to their home airport after a visual flight rules (VFR) night cross-country instructional fight. During the last segment of the flight, as the student was descending toward the airport, the airplane's wheels contacted the terrain on a ridge about nine miles from the planned destination, and about 1,650 feet higher than the airport elevation. Although the airplane initially rolled across the terrain at a high rate of speed, it soon encountered the elevated edge of a dirt road. The contact with the road edge resulted in a loss of control of the airplane and further contact with the terrain. The sequence of events resulted in substantial damage to the airplane's fuselage. Although the sky was clear with no restrictions to visibility, it was a dark night with no moonlight, and neither pilot saw the terrain prior to the accident. Although the instructor pilot did not specifically remember an unusual rate of descent or any sense of the airplane sinking, it was his opinion that a "mountain wave downwash" may have contributed to the accident.