On January 4, 1996, approximately 0745 mountain standard time, a Beech B100, N924WS, sustained substantial damage when it contacted a snow berm during landing rollout at Miles City, Montana. The airline transport pilot, commercial pilot, and three passengers on board the aircraft were not injured. The flight, conducted under 14 CFR 135 with an IFR clearance, had originated at Billings, Montana, about 0655. Instrument meteorological conditions prevailed at the time of the accident. There was no fire, and no report of the ELT actuating. The pilot-in-command was occupying the right seat of the airplane at the time of the accident. The left seat was occupied by the commercial pilot. The pilot-in-command stated that the aircraft struck an 18 inch snow berm along the left side of the plowed area of the runway with the left main landing gear. The aircraft was pulled left off the runway, and the nose landing gear sheared as the aircraft left the runway. The pilot stated that the left third of runway 2 was not plowed. According to police officers, 45.5 feet of the width of the runway had been plowed, with the debris collected in a berm on the left side of the plowed area. No notam had been filed concerning the partial plowing of the runway. The commercial pilot stated that during the weather briefing, the pilot-in- command had been advised of "'thin, loose snow' as a notam 'on the runway.'" A Miles City police officer went to the scene of the accident and measured the plowed portion of the runway and the distance the aircraft travelled off the plowed runway. His diagram and report are attached. The average height of the berm was 17 inches, on both sides of the plowed area. The plowed width of the area was 55 feet at the time he arrived, however, he stated that the snowplow operator had gone out on the runway after the accident and plowed an additional width of the runway. The police officer ascertained that the width of the plowed runway available to the pilots at the time of the accident was 45.5 feet. The aircraft travelled about 260 feet before coming to a rest.