The pilot reported that he and his father had flown to the accident location to camp and hunt. They made three passes over an open area to find a suitable landing spot. The pilot stated that the grass appeared to be knee-to-hip height, but he believed it was safe to land. He set up for landing, and on the landing rollout, about 20-30 knots, the left main landing gear wheel struck an unseen, large embedded boulder that collapsed the left main landing gear. The airplane's wing sustained substantial damage. The pilot reported no preimpact mechanical malfunctions or failures with the airplane that would have precluded normal operation.