On limestone terrain, most rainwater soaks straight into cracks in the bedrock and surface channels rarely develop. The difference on the moor north-west of Pen-y-ghent is that the Lower Hawes limestone bench is coated by a layer of less-permeable glacial boulder clay (drift), which has allowed a small stream network to develop, centred on Hull Pot Beck. That has eroded through the drift down to bedrock, so the beck usually vanishes underground well upstream of this point. However, during periods of heavy rainfall, the infiltration capacity is exceeded and the stream enters Hull Pot over this ~20 m high waterfall.
During extreme sustained rainfall, water can't drain out quick enough either, and the whole basin has been known to flood.
For those keeping count, the streambed and upper walls are of Lower Hawes limestone, the base of the Yoredale series, whereas the lower walls are part of the Malham formation, the uppermost layer of the Great Scar limestone series, which is particularly popular with cavers for its ability to support networks of passages.
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