The pilot of the tailwheel-equipped airplane reported that, during the second approach to the unimproved canyon airstrip in moderate turbulence, he felt that the groundspeed was faster than normal and that he there was a tailwind, but he reduced power to idle and landed the airplane. About 300 ft into the ground roll, at 35 mph, the airplane veered left. He applied right rudder and moderate braking, but the airplane exited the airstrip to the left, and its left main landing gear struck a pile of rocks that were painted white and used to mark the edge of the runway. The left main landing gear separated from the airplane. The airplane sustained substantial damage to the fuselage, left wing, and left aileron. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. An automated weather observation system about 16 nautical miles from the accident site reported that, about the time of the accident, the wind was from 290° at 13 knots, gusting to 16 knots. The pilot was landing to the south.