A raging fire of undetermined origin last night destroyed the Big Boulder Ski Chalet at the popular ski resort in Kidder Township. Flames quickly spread through the two-story wood frame structure, leveling the lodge in less than an hour. The lodge was billed as the oldest public ski resort in the commonwealth. Lake Harmony Fire Chief Al Klitsch said the building was fully involved when firefighters arrived on the scene. The fire was reported at about 5:45 p.m.