Hills contain some thousands of closed hollows known as sinkholes
or dolines. These are generally small but can be up to 20 m in
depth and 100 m or more in diameter. Sinkholes develop by a variety
of methods: collapse, suffosion or solution.
Collapse sinkholes form when an underlying cave passage collapses
forming a depression on the surface above. These sinkholes are
actually quite rare, as caves are generally quite stable. Around
Smitham Hill, there are some examples where collapse of cavities
within the underlying Carboniferous Limestone, aided by leakage
though a thin cover of Jurassic or Triassic mudstone, has propagated
up to the surface creating a sinkhole. Sandpit Hole, near Priddy,
is another example of a collapse sinkhole.
Most sinkholes form by the process of ‘suffosion’.
This is where loose, unconsolidated material including soil, ‘head’,
loess and clay overlies fissures and joints in the underlying limestone,
and material is washed into these fissures and into the caves beneath.
Suffosion sinkholes tend to develop gradually (over months or years)
as the covering sediment slumps into open fissures in the underlying
limestone, creating a void which migrates towards the surface eventually
creating a sinkhole.
Solution sinkholes form by the uneven dissolution of the underlying
limestone, creating a broad saucer-like sinkhole. A good example
is Bishops Lot on the road between Milton, near Wells and the Hunter’s
Lodge Inn in Priddy. Others occur where a stream sinks underground,
creating a blind valley.
a natural process, the formation of sinkholes is often accelerated
or triggered by human actions. Broken land drains, water mains
and sewerage pipes, increased rainfall, storm events, modified
drainage and diverted surface water can all help wash sediment
into the underlying limestone, causing subsidence. There have been
many well documented occurrences of sinkholes forming beneath broken
water mains, unlined storm-water culverts and leaking swimming
Blind valley associated with a stream sink, Tynings Gruffy
Field nature reserve, Charterhouse. The stream sinks beneath the
rocky outcrop on the right of the photograph, reappearing in GB
Cavern. The valley is cut in a sheet of silty loessic clay which
overlies the limestone.
Whitepit, a suffosion sinkhole a short distance to the south
of Priddy. Excavations by cavers revealed a small cave into which
the overlying superficial deposits had subsided.