A Question of Limestone

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A Question of Limestone Click a number to link
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V Vannet
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A Question of Limestone
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What is this feature and how was it formed?
How does it become this?
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Why is there no sign of the river which eroded
this valley?
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Image produced from the OS Get-a-Map service.
Image reproduced with kind permission of the OS
and OS of Northern Ireland
What map evidence suggests the area is underlain
by limestone?
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This is the largest known example in the
UK. What is it and how was it formed?
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This is the top of Britains highest waterfall.
Why is it seen by very few people?
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What features of the structure of carboniferous
limestone are visible in this photo?
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Why is the only limestone visible in this
photograph underneath the big boulder?
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What is happening here? Check out photo 5.
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What are these shallow circular depressions?
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This is limestone pavement. After glaciers had
scoured the rock, it was exposed to chemical
weathering by mildly acidic rainwater. This
enlarged the joints in the rock to
form grykes. The blocks of rock between the
grykes are called clints.
Where rainwater has lain in pools on the clints,
there are solution hollows and where rainwater
has flowed off the clints, there are channels
called runnels.
clint
runnel
gryke
Solution hollows
After prolonged weathering, the pavement will
erode until it resembles the second photo on
Slide 1.
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This is a dry valley. Although it has a V
shaped cross section and was clearly eroded by
water, there is no river flowing in it at
present. At the end of the Ice Age when ice
blocked all the underground passages in the rock,
the limestone became temporarily
impermeable. Meltwater had to flow over the
surface and carved valleys such as Watlowes in
the photo. After the ground thawed, water was
able to make its way underground again and the
valley was left dry.
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intermittent drainage
limestone pavement
swallow hole
gorge
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This is Britains largest stalactite the
so-called Sword of Damocles in Ingleborough show
cave. Stalactites form in underground caves where
water rich in dissolved calcium carbonate drips
from joints intersecting the cavern roof.
Evaporation of the water results in the
deposition of a tiny amount of calcite. This
process, repeated over a very long time, produces
a stalactite.
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When Fell Beck disappears underground at this
swallow hole which is known as Gaping Gill, it
plunges for over 100 metres into an underground
cavern. Fell Beck rises on the impermeable
Yoredale rocks of Ingleborough Hill. It flows
downhill until it meets Carboniferous Limestone.
At this point it disappears underground via the
swallow hole of Gaping Gill. Once underground,
it makes its way through the permeable limestone
until it reappears beside Ingleborough cave on
the impermeable rocks of the valley floor.
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joint
bedding plane
joint
bedding plane
joint
bedding plane
Limestone has cracks which run horizontally
(bedding planes) and cracks which run vertically
(joints). The bedding planes represent periods
of interruption in the deposition of the rock.
The joints formed when crustal movements placed
the rock under stress.
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The large boulder is an erratic which was
deposited on top of the limestone pavement by a
glacier during the Ice Age. It has protected
the limestone underneath and prevented it from
weathering. All around, where the limestone has
not been protected, chemical weathering has
lowered the surface of the pavement. The height
of the plinth of limestone beneath the erratic
represents the amount of weathering which has
occurred since the last ice Age.
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On the May Bank Holiday, the Bradford Pothole
Club sets up a winch at the top of Gaping
Gill. Members of the public can be lowered free
of charge deep underground into a huge chamber
the size of York Minster. It is an awesome
experience. P.S. It costs 8 to be winched back
to the surface!
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These little craters are shake holes and they
are commonly found in limestone country. They
form when percolating rainwater enlarges joints
beneath the surface. Surface deposits (often
boulder clay) fall into the enlarged joints
creating depressions which may range in size from
2m to 15 m across.