Each town, city and borough shall procure its bounds to be set out by plain and durable marks and monuments, which shall be either an iron pipe or rod, projecting at least six inches above the surface of local permanent rocks, or by stone pillars, set at least three feet in and one foot above the ground, at each angle, and once in each eighty rods in the lines running from angle to angle. Such rocks or monuments shall be plainly marked with the initials of the names of the towns adjoining.