Q Why has Weardale got so many waterfalls?
A Its geology. 300 million years ago the North Pennines, which were then located near the equator, were covered alternately by shallow tropical seas and river deltas, resulting in alternate layers of the bones of sea creatures, mud and sand being deposited. These deposits hardened into layers of limestone, shale and sandstone. The shales are worn away more easily by the rivers of Weardale resulting in lips of limestone and sandstone where waterfalls occur.
Q The geology of Teesdale is similar to that of Weardale so why are its waterfalls, like High Force, so much more impressive?
A The Whin Sill. This is layer of very hard igneous rock up to 70 metres thick which lies under much of County Durham and Northumberland. It was produced from magma flows, which instead of reaching the surface as volcanoes, spread out through the rock strata. The Whin Sill occurs at the surface in Teesdale but not in Weardale. Rivers erode the Whin Sill much more slowly than the layers in which it is embedded, producing the high waterfalls of Teesdale.