On September 12, 1998, about 1630 eastern daylight time, a Cessna 180, amphibian, N6503A, was substantially damaged during landing at Mountain Road Airport (MD43), Lakeshore, Maryland. The certificated airline transport pilot and two passengers were not injured. Visual meteorological conditions prevailed and no flight plan had been filed for the personal flight conducted under 14 CFR Part 91. In a telephone interview, the pilot stated that he entered a left downwind for the west runway at Mountain Road. On final, he lowered the landing gear and selected full flaps before clearing the trees on the approach end of the runway. The pilot added that he touched-down smoothly approximately 300-400 feet down the 1,800 foot grass strip. According to the pilot, while "rolling out", and approximately 30 mph, the airplane yawed right about 170 degrees causing the left float to collapse and fold under the airplane. After the float collapsed, the airplane came to rest on a magnetic heading of about 080 degrees, with the engine still running, and the propeller impacting the left float. The pilot secured the engine and all three occupants egressed without injury. After egress, the pilot found the right nose wheel about 40 feet prior to the airplane's final resting spot in an area of loose dirt that measured approximately 12 inches wide and 8 to 10 inches deep. In addition, he found a partially buried boulder between the area of loose dirt and the approach end of the runway. The pilot estimated the boulder's exposed area as 3 to 4 inches high, 12 to 18 inches across, and 18 to 24 inches long. He also observed scratch marks on the top of the boulder. The pilot noted that the boulder and loose area of dirt lay in the path of the wheel and skid marks left by the accident airplane. The Safety Board Investigator examined the right float's nose wheel and observed beach marks, corrosion on the fracture surface, and other signs consistent with a preexisting fatigue crack.