On September 29, 2004, about 1230 mountain daylight time, an amateur-built Arganbright Van's RV-4, N968MA, sustained substantial damage when it went off the right side of runway 26 (5,500 feet by 60 feet, asphalt) at the Custer County Airport, Custer, South Dakota. The 14 CFR Part 91 personal flight was operating in visual meteorological conditions without a flight plan. There were no injuries. The flight originated from Yankton, South Dakota about 1115. The pilot submitted a written report of the accident. He stated that he was landing to the west and during the landing roll, the airplane suddenly veered to the right. He stated that he attempted to correct with rudder and brakes but was unable to keep the airplane from exiting the runway surface. The airplane subsequently struck an embankment. In his statement, the pilot stated that there were no mechanical problems with regard to the airplane. He also stated that he suspected that the winds from the canyon adjacent to the airport caused a wind shift at the point where the loss of control occurred. The reported winds were from 290 degrees at 9 knots.