Sinkholes

Doline formation diagram

Sinkholes (often called dolines) are closed depressions found in karst landscapes. Their sides vary from gently sloping to vertical, forming saucer-shaped hollows or conical or cylindrical potholes or shafts.

In the Yorkshire Dales there are different kinds of sinkholes formed mainly by solution, collapse or suffosion, although in some cases they are formed by a combination of these.

Sinkholes are usually approximately circular in plan and range from a few metres up to about a kilometre across in some parts of the world.


Although sinkholes do not have an outlet for surface drainage, water escapes into the subterranean rivers through joints and fissures in their floors.

Streams or rivers may enter a sinkhole and disappear underground. This type of sinkhole is often called a swallow hole.

Sinkholes are most frequently found in massive Carboniferous limestone landscapes. Buried sinkholes, or pipes, are common in the Chalk. Sinkholes are also common in the Jurassic strata of the Cotswolds, but they are equally common in gypsum bearing strata e.g. Ripon.

 

Solution sinkholes

Doline

Solution sinkholes are formed by local chemical weathering of the limestone where water accumulates around a fissure or joint in the rock. This may be underneath the soil or on the ground surface. The hollow that is formed is drained of water through the fissure or joint, but not before it has dissolved some of the limestone. The depression gradually gets enlarged.

(Note: solution sinkholes also occur in other rock types.)

Collapse sinkholes

Collapse sinkhole

Collapse sinkholes occur where the gradual collapse of a cave passage occurs and eventually causes subsidence at the surface level. Sinkholes formed exclusively this way are quite rare, although many sinkholes are in part formed by collapse: chemical weathering in a solution sinkhole may cause a part of the wall to become unsupported and unstable, resulting in collapse.

Collapse sinkholes are not rare where limestone is overlain by sandstone.

Collapse sinkholes may form quarry-like depressions like Hull Pot.

 

 

Suffosion sinkholes

Suffosion sinkholes

Suffosion sinkholes form where solution of the limestone has created a depression on the surface of the limestone, but under a covering of soil. The unsupported soil subsides into the cavity and leaves a depression in the landscape. These are sometimes referred to as subsidence sinkholes and in Yorkshire, where they are particularly common, they are often known as 'shake holes'.

The glacial tills of Malham Moor are pockmarked by suffosion sinkholes, between about one and 15 m across. Some still have soil and grass on the floor of the sinkhole, but in others the fissured limestone is exposed, the soil having been eroded away by the rain and washed down into the underground river system.