A fire off Islands Highway in east Liberty County Sunday caused thousands of dollars in damage to heavy equipment used to uproot and grind trees.   According to Liberty County Fire Chief Brian Darby, the fire was reported by a passing motorist who spotted flames and heavy smoke.  When firefighters from Liberty County's Eastern District Volunteer Fire and Rescue Department arrived, they encountered a blaze that was 200 feet in diameter. Flames from burning wood stacked 15- to 20-feet high shot into the air. Two pieces of heavy equipment used in processing wood was damaged in the fire. The equipment was valued at $125,000 dollars. No one was injured in the fire. Firefighters were able to douse the fire around the equipment while the remainder was allowed to extinguish on its own. The cause of the fire is unknown.