The removal from any property of earth products, including but not limited to sod, loam, sand, clay, silt, muck, peat, gravel or quarried stone, shall be prohibited in all districts, except when incidental to and in conjunction with the constriction of a building or other activity authorized by this bylaw as interpreted by the Building Inspector, and except such removal may be authorized through the issuance of a special permit by the Planning Board. All existing commercial excavation facilities (those which have been previously authorized by permit by the Board of Selectmen or Planning Board) shall be considered nonconforming uses and shall be subject to the provisions of this Zoning Bylaw, § 5.1.