Swallow Hole/Resurgence/Shake Hole
A stream flows over impermeable rock like Millstone Grit, but when it starts to
flow over permeable rocks like Limestone it starts to flow underground.
Permeable means there are cracks, joints & bedding planes, in the rock which
allow water to drain down through it.
Impermeable means there are no cracks in the rock so water flows over the
surface of the rock. e.g. Malham Moor
The water flows through the limestone as a river until it eventually meets a
layer of Millstone Grit beneath it. At this point it flows across the Millstone
Grit until remerges out the side of a hill as a stream. This point is called a
resurgence e.g. Malham Cove
A shake hole is when impermeable sediment in the river is deposited in a
swallow hole. This effectively blocks up the hole making it impermeable leaving
a depression on the land surface. e.g. Malham Moor
Millstone Grit
Swallow Hole
Bedding Plane
Resurgence
Joint
Limestone
Cave Systems & Dripstone Features
Caves, as natural cracks or tunnels (joints & bedding planes) in limestone, are
widened to the point of collapse. This process is the result of water action,
which erodes the tunnels at their base. This causes the sides to undercut,
overhang and then collapse as there is no support for the walls creating a cave.
Water with dissolved limestone (Calcium Hydrogen Carbonate) in it travels
through joints and bedding planes in limestone. As it does so, it comes into
contact with CO2 in the air, whereby a chemical reaction takes place depositing
limestone. There are many impurities in the limestone deposits and so it is
called calcite rather than limestone. Any feature made from these deposits is
called a drip stone deposit.
As water drops slowly evaporate from a cave roof they leave behind deposits of
calcite, which hang down from the ceiling like icicles of rock. These tend to be
quite angular in appearance, called stalactites.
When drops of water hit the floor of the cave they leave behind calcite. This
time the rock builds upwards, though is wider and rounder than a stalactite due
to the water droplet spreading when it hits the cave floor, called stalagmites.
A pillar simply forms from the above two processes joining together, so have
more angular tops and rounded wider bottoms. It is the rarest of the three
dripstone deposits. All these dripstone features grow slowly around 7mm a
year.
Stalactite
Stalagmite
Pillar
Gorges
There are two different theories on how limestone gorges develop; though in
reality it is probably a bit of both.
The first is due to solution and water erosion affecting underground tunnels
(joints & bedding planes) in the limestone. At the base of the tunnel walls,
these processes undercut the wall leading to overhanging walls, which
eventually collapse due to the lack of support. If this happens repeatedly a
vast cavern develops, which if close to the surface, means that the roof will
eventually collapse leaving a gorge.
A second idea is that a very fast flowing river eroded the gorge. This is
thought to have been taken place around 10,000 years ago during the last ice
age. The ground would have been permanently frozen and so the natural joints
of the limestone would have been plugged with ice. As the glaciers melted vast
amounts of fast flowing water would have been able to cut the gorge. The ice
plugs would have remained for a period of time as they would be underground
and so protected from the sun.
Erosion Of Tunnels
Collapsed Cavern
Or Gorge
Dry Valleys
Bedding Planes
Joints
Ice Plugs
A dry valley can form in one of two ways, both requiring a river to flow over
limestone, which because of its permeability means it can’t support rivers.
Permeable mean it has many joints and bedding planes through which water
drains, meaning no rivers on the surface.
The first theory suggests that during the last ice age the ground became
permanently frozen, this effectively plugged the joints and bedding planes of
the limestone making it impermeable. When meltwater flowed over the
limestone it was able to erode a v-shaped valley. As the ice plugs were
underground they took longer than surrounding ice to melt and so were still
present despite surface ice melting. When the ice plugs eventually melt due to
warmer weather, they make the limestone permeable again and so the water
that was once in the valley drains away; as there is a lower supply of water once
the ice has melted it is easier for water to drain through limestone than over
it.
The alternative theory is that during a period of heavier rainfall the limestone
became saturated so that water flowing over it couldn’t drain away. This meant
that rivers flowed over its surface eroding a v-shaped valley. When the climate
became drier the limestone became less saturated and so allowed surface
water to drain through it again leaving a dry v-shaped valley.
Limestone Scars
Cliff
10 – 20m
high
Angular
Scree
Larger Stones
Low Down
These are long cliffs found in Limestone areas e.g. Twistleton Scar in the
Yorkshire Dales. They initially form as glaciers erode a U shaped valley,
bulldozing rock spurs away as it moves and forming these cliffs. They are then
modified after glaciation by freeze-thaw weathering. This is when water gets
into a crack in rock and then freezes. The resulting ice exerts an internal
pressure on the rock as it has a volume 7% greater than that of water. The ice
then melts due to warmer temperatures, but if the process repeats it will
eventually force the rock apart into small stones. For this to be an effective
process the temperature needs to fluctuate around 0°C in order to give
constant freeze-thaw conditions. The rocks fall of the cliff at a rapid rate
gathering at its base forming a scree slope. The larger rocks are found at the
bottom of the scree slope as they bounce down further than the smaller rocks
due to momentum. The actual fall or flow of rock downslope doesn’t require
water, although initially it is required for freeze-thaw to take place.
OS Map Work
Limestone Pavement
Swallow Hole
Resurgence
Dry Valley
Scar
(A)
(D)
(F)
(E)
(B,C)
These are the only landforms
that you will be asked to
find on an OS map.
Limestone Pavement & Scar
200
A
A Outcrop Symbol
B
B Cliff Symbol
C Scree & Close Contours
150
Swallow Hole Resurgence
Dry Valley
D River Stops
200
150
C
D
E
E Contours Point Uphill
F River Starts
F
LimestoneF Pavement - flat area of exposed limestone rock, shown by spaced
contours and outcrop symbols.
Swallow Hole - where a river starts to flow underground, shown by a blue
winding river which suddenly stops. It will cross contours which point uphill in
an inverted v shape.
Resurgence - where an underground river emerges, shown by the start of a blue
winding line flowing over inverted v shaped contours. N.B. There must be a river
which has disappeared above the resurgent river!
Dry Valley - a river valley with no river in it, shown by a series of inverted v
shaped contours indicating a river valley but with no blue winding line, or river,
in it.
Scar - a long cliff, shown by a black line with black “eyelashes” pointing
downhill, contours will be packed together indicating steep land and black dots
below the cliff representing scree.