As the river Dane flows out of the Pennine hills it crosses over either SANDSTONE, which is very hard, or SHALE, which is much softer. The sandstone is too hard for the river to cut through and acts as a cap rock at the top of the waterfall. The shale is easily eroded and washed away creating a PLUNGE POOL at the bottom of the waterfall and undercutting the sandstone.

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