Lundy is the largest island in the Bristol Channel, lying 19 km
off the coast of Devon, approximately one third of the distance
across the channel between England and Wales. It measures 5km x
1.2km which gives it an area of 445 hectares.
Lundy, although a small island, has attracted and intrigued
geologists since the nineteenth century, as it is formed from rocks
that are of a very different type and age to the nearby rocks of
Devon and South Wales. It is formed mainly of granite (a
coarse-grained igneous rock, composed of the minerals feldspar and
quartz) intruded into metamorphosed sedimentary rocks.
The Lundy granite is similar to the granites of Devon and
Cornwall, which were emplaced in Carboniferous times (362-290
million years ago) and was originally thought to be the same age.
However, radiometric dating of the Lundy granite showed that it was
much younger, having a Tertiary age (65-62 million years ago). This
makes it part of the British Tertiary Volcanic Province, mainly
located in Scotland and Northern Ireland. Lundy is therefore of
special interest in that it is the southern-most part of the
Province. It is likely that Lundy was formed as part of a volcano,
which would have been one of the most spectacular geological events
in this area.
The Devil's Limekiln is a sea cave (also known as a littoral
cave) and is a type of cave formed primarily by the wave action of
the sea. The primary process involved is erosion. The Devil's
Limekiln is also open to the air at it's inner end, following the
collapse of it's roof and is an example of a littoral sinkhole.
Littoral caves may be found in a wide variety of host rocks,
ranging from sedimentary to metamorphic to igneous, but caves in
the latter tend to be larger due to the greater strength of the
host rock.
In order to form a sea cave, the host rock must first contain a
weak zone. In metamorphic or igneous rock, this is usually a fault
or a dike. In sedimentary rocks, this may be a bedding-plane
parting or a contact between layers of different hardness. The
latter may also occur in igneous rocks.
The driving force in littoral cave development is wave action.
Erosion is ongoing anywhere that waves batter rocky coasts, but
where sea cliffs contain zones of weakness, rock is removed at a
greater rate along these zones. As the sea reaches into the
fissures thus formed, they begin to widen and deepen due to the
tremendous force exerted within a confined space, not only by
direct action of the surf and any rock particles that it bears, but
also by compression of air within. Adding to the hydraulic power of
the waves is the abrasive force of suspended sand and rock. Most
sea-cave walls are irregular and chunky, reflecting an erosional
process where the rock is fractured piece by piece. However, some
caves have portions where the walls are rounded and smoothed,
typically floored with cobbles, and result from the swirling motion
of these cobbles in the surf zone.
Rainwater may also influence sea-cave formation. Carbonic and
organic acids leached from the soil may assist in weakening rock
within fissures.
Sea cave chambers sometimes collapse leaving a littoral sinkhole
(of which the Devil's Limekiln is an example). Small peninsulas or
headlands often have caves that cut completely through them, since
they are subject to attack from both sides, and the collapse of a
sea cave tunnel can leave a free-standing "sea stack" along the
coast.
Life within sea caves may assist in their enlargement as well.
For example, sea urchins drill their way into the rock, and over
successive generations may remove considerable bedrock from the
floors and lower walls.
Most sea caves are small in relation to other cave types.
Several factors contribute to the development of relatively large
sea caves. The nature of the zone of weakness itself is a factor,
although difficult to quantify. A more readily observed factor is
the situation of the caves entrance relative to prevailing sea
conditions. Caves in well-protected bays sheltered from prevailing
seas and winds tend to be smaller, as are caves in areas where the
seas tend to be calmer. On Lundy Island, the Devil's Limekiln is
situated on the South West peninsula where it is more prone to
attack from the prevailing weather.
The type of host rock is important as well. All of the largest
sea caves are in basalt, a strong host rock compared to sedimentary
rock. Basaltic caves can penetrate far into cliffs where most of
the surface erodes relatively slowly. In weaker rock, erosion along
a weaker zone may not greatly outstrip that of the cliff face.
Finally, caves that are larger tend to be more complex. By far
the majority of sea caves consist of a single passage or chamber.
Those formed on faults tend to have canyon-like or angled passages
that are very straight. By contrast, caves formed along horizontal
bedding planes tend to be wider with lower ceiling heights. In some
areas, sea caves may have dry upper levels, lifted above the active
littoral zone by regional uplift.
Sea caves can prove surprisingly complex where numerous zones of
weakness converge. When caves have multiple entrances, they are
exposed to more wave action and hence may grow relatively
faster.
In order to log this Earthcache, please answer the following
questions and e-mail us the answers:
1) How wide is the opening of the cave at the 'island end'?
2) How high above sea level is the opening of the cave at the
'island end'?
You might like to use your GPSr to help you. Whilst it is NOT
neccessary to upload a photograph of your visit in order to log
this Earthcache, we would love to see photos of you with your GPSr
at the cave opening!
Please take
care when walking near the cave opening!