|
|
| Weathering and slopes.
|
|
When rocks are exposed at the surface they are weathered
(broken down). Weathering is divided into two groups of
processes - mechanical (physical) weathering and chemical
weathering.
Mechanical weathering breaks rock into fragments, whilst chemical
weathering causes the rock to be changed. Both reduce the rock to
loose pieces, which are known as regolith. When the
regolith becomes mixed with plant debris (humus) it forms soil.
Any loose material on slopes will travel down-slope,
under the influence of gravity. This transport process is called mass-movement.
Mass-movement can be divided into several different types - known as creeps,
flows, slides and falls.
|
 1.
During the ice age, the summit ridges of the mountains
of Britain were
exposed above the glaciers. The extremely cold conditions allowed
water to penetrate the cracks in the rock in summer, and frost to break up the
rocks in winter, leaving the slopes littered with jagged boulders of broken
rock. This process is called frost shattering. This
is a mechanical weathering process.
It was once important in
Britain, but is no longer active. Why not ?
This view is on the famous Glyder ridge in Snowdonia.
|
|
 2.
These cracks
have been formed in the clay of Flamborough Head. Clay absorbs water when it gets wet, causing it to expand. When it
dries the rock contracts, causing cracks to form. Slowly the rock
is broken up by repeated wetting and drying. This is another
mechanical weathering process.
|
|
 3.
In the Yorkshire Dales, many of the hills
are made of hard Carboniferous limestone. The
weathering process which has created this landscape is called carbonation.
Weak acids in the rainwater have dissolved the limestone, and it has
been removed by running water to leave these spaces.
This
feature is known as a limestone pavement. How did it get its name ?
|
4.
This
feature is known as a limestone pavement. The water
soaking through the 'joints' in the limestone have dissolved some of
the rock to leave a pattern of open cracks called 'grykes'
separating blocks of rock called 'clints'. The
grykes are a sheltered habitat that allows some specialised plants
like ferns to grow. This is one of the reasons that limestone
pavements are protected by law.
|
|
 5.
This is Burbage Brook in
Derbyshire. The orange stain in the water is caused by iron
oxide seeping into the river from the rocks. Iron oxide is
formed by oxidation of the iron-rich minerals in the
rock. This is a chemical weathering process.
|
6.
Granite is a resistant rock with an uneven pattern
of joints. This gives granite scenery its characteristic jagged
appearance.
This is part of the coastline of the Scilly Islands.
|
|
 7.
The chemical weathering which affects Granite is called hydrolysis.
The minerals of this hard rock were formed from cooling lava.
Now that the rock is exposed in the open air, the minerals are not stable at
surface temperatures. They react with water and change into clays and sands.
This is
the highest point of Dartmoor in Devon.
|
|
 8.
Here on Bodmin Moor (in Cornwall) we can see the result of the
weathering of Granite. The fine sands and clay have been washed
away to expose the granite. This forms a rocky outcrop known as
a 'tor'. Surrounding it are the smooth slopes covered with clay
and sand debris.
|
|
 9.
One product of the weathering of granite is called
'china clay' (or Kaolin). For years it has been the raw material
of the pottery industry and is used to 'glaze' paper. It is quarried on the moors around
Cornwall and Devon.
This is a spoil tip of waste dug out of a clay-pit.
|
|
 10.
In areas of steep slopes, rock-falls are quite common. Here in
on the slopes of Scafell Pike in the Lake District, rocks loosened by
frost action have fallen to form ramps of debris. These are
known as scree slopes. Look carefully at the slope on the
left - Scree slopes are almost straight (have the same gradient)
because all the rock fragments are balanced at the angle of rest of
the material.
|
 11.
These cliffs are near Filey in N. Yorkshire. The
soft clay rock has been undercut by the sea, resulting in a landslide.
A large part of the cliff has collapsed onto the beach. Mass
movement is an important process where slopes are being undercut and made
unstable by the action of rivers or waves.
|
|
 12.
This is a view of the South Downs in Sussex. The rocks here are gently
tilted beds of chalk. The slope to the left of the hill is called a scarp.
The gently tilted slope to the right is called the dip-slope.
Scarp-and-dip landscapes are common in areas where the rock beds are tilted.
|
 13.
This view was taken near the last, on the South
Downs. On the dip slope of the chalk there are dry valleys.
They were formed during the ice age when there were surface streams. Chalk
is a permeable rock, so now there is no surface water on the Downs.
This area used to be important for sheep farming, now it has been ploughed
to be used for cereals. Ploughing accelerates soil creep.
|
|
 14.
These slopes are found in the Brecon Beacons. The sandstone rock has
been weathered, and thin soil covers the slope, allowing coarse grass to
grow. This is called a mantled slope. The warm wet climate of
Britain encourages the development of mantled slopes.
|
|
 15.
These are the cliffs at Folkestone on the South Coast. The chalk of
the South Downs lies on top of beds of clay and sand. Water soaking through the chalk has waterlogged the underlying sediments. Where they are undercut by the sea there have been landslides.
|
|
Return to the top / Return to
the home page
|