The Porter Brook, like the other rivers in Sheffield, is ideally suited for providing water power, as the final section falls some in a little over . This enabled dams to be constructed reasonably close together, without the outflow from one mill being restricted by the next downstream dam. In addition to this water-power, natural sandstone was available in abundance in the nearby hills of Sheffield and the Peak District National Park. Lying at the edge of these hills, mill-owners found Millstone Grit was an easily extractable resource. By 1740 Sheffield became the most extensive user of water-power in Britain and probably in Europe. Ninety mills had been built, two-thirds of them for grinding. By 1850 these mills numbered well over 100. In the Porter valley alone 21 mill dams served 19 water-wheels, mostly used for grinding corn, operating forge-hammers, and rolling mills, grinding knives and the various types of blade that made Sheffield famous. Most were active during the 18th and 19th centuries.