Due to the large size and amount of building material need to construct these dams, the arch dam was invented. An arch dam is dependent on its shape for strength, requires less material to build, and is relatively thin. The first known arch dam is Kebar, which was built around 1280 AD in the Mongol period. The limestone dam is located near the ancient town of Quam and stands 85 ft high, 180 ft long at the crest, 16 ft thick at the crest and has a constant radius of curvature of 125 ft. An arch dam needs to be supported by the surrounding geology, the rock formations on either side support the arch.