Ironclads were the forerunners of future generations of armored naval vessels. Instead of the simple wooden hulls found on the other ships at the time, ironclads were covered with strong metal plating. This plating made them nearly impervious to large-caliber explosive shells that would easily penetrate and destroy wooden vessels. Ironclads made their first appearance during the American Civil War, and quickly gained dominance over other ships of that era. Perhaps the most famous naval battle of the Civil War was the battle between the Confederate ironclad "Merrimack", and the Union ironclad "Monitor", which lasted for several hours before the "Merrimack" was forced to withdraw.