On March 29, 1994, about 1545 central standard time, a Cessna 170, registered to C.P.T., Inc, was substantially damaged during a forced landing in a field about 5 miles east of the Olive Branch Airport, Olive Branch, Mississippi, while on a 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The private-rated pilot and one passenger were not injured. The flight originated from the Olive Branch Airport about 1400. The pilot stated that the flight was about 5 miles from the airport returning to land and before descending, he reduced the power only. After descending, he applied throttle to increase engine power but the engine did not respond. He attempted a forced landing in a field, and during the landing roll the airplane collided with trees and a fence. Examination of the airplane by an FAA airworthiness inspector the following day revealed that the left wing fuel tank was damaged. The right wing fuel tank and gascolator which were not damaged were checked for fuel contamination; about 1/2 ounce of water was present in both. The fuel selector was in the "both" position and had not been moved from that position after engine start before departure. According to an icing probability chart, conditions were favorable for visible icing at rated power with a float type carburetor installed.