Manufacturing, processing or assembling of products, except that after September 1, 1990, no rock, sand, gravel or other earth products which are excavated outside of the Town of Beacon Falls shall be screened, washed, crushed or otherwise processed in an industrial district, provided that stockpiles of earth products excavated outside of the Town of Beacon Falls which are in existence as of September 1, 1990, must be entirely processed as of September 1, 1991, subject to the requirements of Article 64.