Swallow holes

Swallow holes are sinkholes down which a river or stream disappears via a fissure or shaft to join the subterranean drainage system below. Downstream of the swallow hole, a dry valley may occur.

Swallow holes are found in different kinds of limestone areas, including chalk (e.g. in the Chilterns and North Downs), although they are small.

In the Yorkshire Dales, however, swallow holes may form wide, vertical shafts known as potholes, leading to extensive cave systems. Perhaps the most spectacular is Gaping Gill near Ingleborough, but there are a number of these in Yorkshire, including Hunt Pot and Alum Pot.

Hunt Pot

Hunt Pot

A small stream flows into Hunt Pot, which is an elongated sinkhole a few metres wide but about 15 m long. The stream falls down a 30 m high waterfall into a cavern below.

Alum Pot

Hunt Pot

Alum Pot is an example of a swallow hole that was partly formed by chemical solution and collapse, and partly the result of erosion by the river. The shaft is about 70 m from the ground surface to the floor. A major joint passes through the limestone here, and water has picked out this line of weakness. Over a long period of time, the water widened the joint by dissolving the limestone. This developed weaknesses on one wall of the shaft, which collapsed, and a huge block of stone fell, partially blocking the shaft and now forming a bridge across the chasm.

The main river draining Simon Fell has found another route, so only a small stream now falls down the Alum Pot swallow hole. It is often the case that when a new swallow hole develops the original one is abandoned by the river.