The beauty, grandeur and
variety of the scenery strike all visitors to the Lake District, but few
appreciate the extent to which this has been influenced by the underlying rocks,
and the natural processes shaping their surface. Yet the geology is there for
all to see, in the form of crags, road cuttings, and rocky knolls in abundance.
Evidence of man's exploitation of geological materials is also widespread, with
numerous abandoned mines and quarries, and a now growing number of active
quarries. There are hundreds of farm and village buildings and miles of walls
made from local stone. Most striking of all, there is the magnificent contrast
between fell and dale, the result of glacial action during the last few thousand
years.
Distribution and Origin of the Rocks of the Lake District
The bulk of the National Park is made up of
three broad bands of rock trending from SW to NE. The rocks, which form the
mountains and hills, are not necessarily `harder' than those, which form
adjacent lower land. The Lake District rocks are buoyed up by sitting `astride'
a batholith or `raft' of low-density granite. The granite underlies the whole
area and small protruding parts of it are seen at the surface as the Eskdale,
Ennerdale, Skiddaw and Shap granites.
The SKIDDAW GROUP is the oldest group of
rocks in the Lake District. They were formed during the ORDOVICIAN period, about
500,000,000 years ago, as black mud settling on the seabed in relatively deep
water, where occasional layers of coarser slit and sand were also deposited.
The Skiddaw Group forms a roughly triangular
mountainous zone in the north of the Park, reaching a maximum height of 931m on
Skiddaw itself. Despite their slaty appearance, they are not suitable for
roofing because they easily break into small pieces, and although there are some
rugged areas, especially in the cirques such as those on the south face of
Blencathra, the mountains they form are mostly smooth.
South of Skiddaw Slates lay the rocks of the
BORROWDALE VOLCANIC GROUP (B.V.G), which formed later in the ORDOVICIAN, about
450,000,000 years ago. These rocks are very hard lavas and pyroclastics erupted
to form a large volcanic island. The chief lava is ANDESTINE, but there are also
BASALTS and RHYOLITES. The volcanic products (such as ash) range from very
fine-grained TUFFS to very coarse AGGLOMERATES.
The B.V.G rocks underlie the highest and
craggiest central part of the Lake District, including the well know peaks of
Scafell (964m) and Scafell Pikes (978m), Helvellyn (950m), Coniston Old Man
(803m) and the Langdale Pikes (736m). The ruggedness of the terrain makes this
the most popular area with fell walkers and rock climbers.
Further south again is a zone of slates,
siltstones and sandstones, also formed in the sea during the SILURIAN period,
about 420,000,000 years ago. These rocks are known to geologists as THE
WINDERMERE GROUP.
These rocks are less resistant than the B.V.G
and form a belt of foothills stretching from the Duddon estuary to Kendal, and
are possibly the most familiar part of the Lake District to many visitors as
they lie across the A591, the main tourist route into the National Park, and
including as they do Lake Windermere and Coniston Water.
About 400,000,000 years ago, all these rocks
were folded, faulted, intruded by molten magma, and pushed up to form a very
high mountain range. These events are known as the CALEDONIAN OROGENY, and they
were caused by the collision of two continents. The modern Himalayas, formed by
India colliding with Asia, give a good idea of what these mountains originally
looked like. The Caledonian Orogeny compressed many of the B.V.G. tuffs and
turned them into the famous Westmorland green slates.
Millions of years of erosion have worn down
these mountains to their present size, but the folds and faults can still be
seen, and the igneous intrusions, which cooled down hundreds of metres below the
surface, are exposed around Eskdale, Ennerdale, Shap, Skiddaw and Carrock Fell.
They all form part of a very large granite `batholith' at depth (as already
mentioned) and this tends to `buoy up' the whole area.
During the DEVONIAN period, the high
mountains were eroded to low hills, and about 350,000,000 years ago, the land
sank beneath a tropical sea. This teemed with life, and the sea floor became a
thick layer of sediment made up of the broken remains of shells to form the pale
grey Carboniferous Limestone. Some shells survived intact, and so fossil corals,
brachiopods and snails are sometimes very common.
During the latter part of the CARBONIFEROUS
period, this sea was eventually filled in with mud and sand, and was colonised
by swampy forests whose remains now form coal. These rocks, however, lie just
outside the National Park boundary. About 280,000,000 years ago, the
Carboniferous rocks were uplifted and folded into a broad dome by another
orogeny, the HERCYNIAN OROGENY, whose most severe effects were felt in what are
now Devon, Cornwall, South Wales and South West Ireland.
The top of this dome has long since eroded
away, leaving the Carboniferous Limestone as a broken rim girdling the higher
mountainous core. The limestone takes the form of a long, curved cuesta around
the northern edge of the National Park, and a more broken zone in the south
lying only partly within the Park. It contains two very prominent west-facing
scarps, Whitbarrow Scar and Scout Scar; both situated to the west of Kendal.
Since the formation of the Skiddaw Group
during the Ordovician, the part of the Earth's crust we now call the Lake
District had been slowly drifting north, starting from a position well south of
the equator. During the late Devonian or early Carboniferous, it crossed the
equator, and by about 250,000,000 years ago, it reached the latitude of the
present day Sahara Desert.
A landscape of sand dunes and salt lakes
developed, with seasonal downpours of rain-washing rock debris from nearby
uplands onto stony plains. These conditions persisted throughout the PERMIAN and
TRIASSIC periods and ended about 190,000,000 years ago.
The St. Bees and Kirklinton Sandstones to the
west and north of the Park were formed at this time and are thought to be the
result of `flash floods' whereby a mass of sand or larger debris is deposited in
a sudden downpour or storm.
By contrast, the Penrith and Lazonby
Sandstones of the Eden Valley (east of the Park), although deposited in the same
areas, are true desert sandstones showing dune bedding and rounded (aeollan)
grains, which reveal their history. Since the formation of the Triassic rocks,
many more rocks have been formed in what is now the British Isles, but none of
them are now found in the Lake District. The drift northwards, however,
continued, bringing us to our present latitude.
About 2,000,000 years
ago, the Earth's climate cooled, and glaciers formed in the mountainous areas
such as the Lake District, eventually spreading over most of the rest of the
country. During warmer spells, the ice melted, only to return when the climate
cooled again.
We are presently
living during one of the warmer periods called INTERGLACIALS, so it is possible
that the ICE AGE has not yet finished. It is the action of glaciers and melt
water, of
frost and tundra conditions, which have shaped all the present
surface detail of the Lake District, and made the area a textbook example of
such landforms.
Examples of Glacial
Features around Helvellyn
