A demonstration of the features formed by water drainage in limestone areas, both at the surface and underground. Permeable and impermeable layers of rock in the Malham area give rise to intermittent drainage. We see features formed both at the surface and underground. Surface water passes over impermeable rock until it reaches permeable limestone. The water passes through the limestone and erodes it to form swallow holes. Gaping Gill is a swallow hole where water has eroded a cave to form the highest waterfall in Britain. The water continues to flow underground until the limestone meets impermeable rock at the valley floor. Here there is a resurgence where the water returns to the surface. Shake holes and water sinks are also formed by the flow of surface water.

First broadcast:
17 February 2001

Students could produce annotated diagrams explaining the formation of the drainage features shown. Provide a map with the names of features removed and have students correctly identify each feature. Compare these maps to geological maps to see where the features formed are linked to the boundaries of different rock types.