The Gothic church spire originated in the 12th century as a simple, four-sided pyramidal structure on top of a church tower. The spire could be constructed of masonry, as at Salisbury Cathedral, or of wood covered with lead, as at Notre-Dame de Paris. Gradually, spires became taller, slimmer, and more complex in form. Triangular sections of masonry, called "broaches" were added to the sides, at an angle to the faces of the tower, as at St Columba, Cologne. In the 12th and 13th centuries, more ornament was added to the faces of the spires, particularly gabled dormers over the centres of the faces of the towers, as in the southwest tower of Chartres Cathedral. Additional vertical ornament, in the form of slender pinnacles in pyramid shapes, were often placed around the spires, to express the transition between the square base and the octagonal spire.