The bridges in this pack are paper, but they look like stone once you have cut out and assembled them. There are one 3-arch bridge, two 2-arch bridges and two 1-arch bridges included in the set. Unlike most of our building models, these bridges are designed to have a reinforcing layer of foamcore or balsa wood in the side supports. The foamcore or balsa is not included in the kit and must be provided by the buyer. The 15mm bridges have a 2" wide roadway and are 5", 7" or 10" long depending on the number of arches. The 10-12mm set has a roadway 1 1/2" wide with the bridges being approximately 3 1/2", 5" and 7" long. The 20mm set has a roadway that is 2 2/3" wide with the bridges being 6 1/2", 9" and 12" long. The bridges come in two different color stones: brown and gray. You may select all brown, all grey or a mix.