The steady destructive forces of Ice Age glaciers left distinctive stepped valleys. Most notably in Swaledale, the valley floor was scoured by slow moving glaciers in the last Ice Age (22,000 years ago at its maximum) which removed surface deposits and the valley spurs creating u-shaped valley profiles. Where the Yoredale Group of rocks have been exposed, the limestone beds interspersed with mudstones give perfect conditions for the formation of waterfalls. The weaker mudstones are easily eroded and their removal by the river has caused the underlying limestone beds to undercut, leading to collapse. This results in a series of steps and waterfalls, along the length of Swaledale and its tributary valleys.