Field Studies: The Geological History of the Lake District
N
St John’s Vale
Carlisle
Irish
Sea
Penrith
Keswick
30 Km at front.
These are geological maps of the Lake
District. The first indicates a few of the
major towns and cities in the area and the
second shows the location of St John’s
Vale.
Grasmere
100 Km
Ordovician
The oldest rocks found in the Lake District are of Ordovician age (495-443Ma). The paleogeography of the
Ordovician age was that of orogenises, namely the Caledonian Orogeny. This occurred when Avalonia broke free
from Gonwanaland and began drifting northwards towards Laurentia, closing the Iapetus Ocean. The rocks found at
this time are called the Skiddaw Group and are represented by at least 1000m of mudstones and greywakes deposited
on a sinking sea floor. As the Iapetus Ocean began to close another significant layer of strata was deposited; The
Borrowdale Volcanic Group (BVG). These consist of lavas and pyroclasts, which erupted above a subduction zone,
forming an island arc. The chief lava is andesite, but there are also basalts and ryholites present. Volcanic products
range from very fine grained tuffs to coarse agglomerates.
Fig 1. This a plan geological map view of the Lake District.
N
Location: Hard Knott, Eskdale. View looking
N. Description: Borrowdale Volcanic Group.
Thick sheets of welded acid ignimbrite form
much of the rugged scenery of the Central
Fells of the Lake District. On Hard Knott,
seen here in the foreground, volcano-tectonic
subsidence has juxtaposed a block of this
sequence against andesite and basalt much
lower in the stratigraphy. Well featured here
within the ignimbrite sequence are massive
boulder conglomerates capped by bedded
pebble and cobble conglomerates derived from
rapid erosion of the ignimbrite sheets.
Photographer: McTaggart, F.I. Geologist:
Millward, D. SW Corner: NY23200240 Sheet
No: England & Wales 38 County: Cumbria
Year Taken: 1988
Major
Volcanic
Intrusions
(red)
probably
all
connected
to a large
batholith.
The BVG (Purple). Note the
close proximity to major
igneous intrusions
The Skidddaw
Group (Green)
100 km
Silurian
Further south lie rocks of Silurian age (443-417Ma). This was a time of much less volcanic activity and the
rocks of the Lake District represent about 4600m of greywakes, sandstones and graptolitic shales following
conformably on the Ordovician rocks. These formed in a relatively shallow sea and probably formed at a time of
decreasing tectonic activity. These are known as the Windermere Group. These are less resistant than the BVG and
form belts of foothills stretching from Duddon Est. to Kendal. About 400ma these rocks were folded, faulted and
intruded by a large molten magma chamber to form a very high mountain range. This indicates regeneration in
tectonic activity and therefore a continuation of the Caledonian Orogeny. The Caledonian Orogeny compressed much
of the tuffs, changing them to the Westmorland Green Slates.
Devonian
The Devonian was a time of increases weathering and erosion. Due to the high orogenic belt created in the
Lake District, this provided a source material for weathering and erosion to occur. Therefore we observe no Devonian
rocks in the Lakes as that period was a time of heightened erosion rates and so none were deposited.
Topographic profile of the
Lake District. White indicates
high relief i.e. antiforms and
the black indicates valleys.
Note: The large degree of
faulting (purple lines) was
probably due to the
Caledonian Orogeny.
Faulting probably occurred in
both the Ordovician and
Silurian with the closing of
the Iapetus Ocean
compressing the Eastern side
of Laurentia
The Windermere
Group. Note the
folding of the area due
to the Caledonian
Orogeny.
Keswick
N
23km
Fig 2. This map looks North East towards the Pennines.
Carboniferous
Next in the stratigraphic succession came rocks of the Carboniferous age (354-290Ma). At this time Britain lay
on the equator and in Carboniferous times a warm, shallow sea covered most of Britain, interrupted by islands of high
altitude. This shallow sea teamed with life and the sea floor became a thick layer of sediment made up of the broken
remains of fragmented shells to make up a pale grey Carboniferous limestone. Some shells survived intact so fossil
corals, brachiopods and gastropods are common as fossiliferous limestones. During the latter part of the
Carboniferous, this sea was eventually filled in with deltaic muds and sands as this sea began to gradually infill. This
swampy deltaic area was then colonised by tropical forests, whose remains now form coal. The coal seams created
however are beyond the National Park Boundary.
Carboniferous – Permian Transition
During the transition between the Carboniferous and Permian, the Lake District became a victim of the
Variscan Orogeny. This orogenic belt ran through Central Europe and across to North America. It occurred when
the Tethys Ocean closed, when Gonwanaland collided with Laurasia to form the supercontinent Pangea. The south of
Britain was more effect by this orogenic belt; however the weaker effects of the orogeny were felt in the north.
Approximately 280 Ma at the beginning of the Permian the rocks of the Carboniferous were uplifted and faulted into a
broad dome. The top of this dome has since been eroded away, leaving the Carboniferous rocks exposed as a broken
rim girdling the higher mountainous core. The rocks take 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.
100km
Carboniferous rocks
(turquoise / grey).
This shows the distribution of
Carboniferous rocks. You can
clearly see that they are circular in
shape due to erosional effects.
Pennine Faults. These were
an effect of the Variscan
Orogeny.
N
5km
Fig 3. These maps show the distribution of Carboniferous rocks and
give good indication of the doming that occurred during the
Variscan Orogeny.
This picture shows the heavily folded Carboniferous Limestone. The
obvious gaps or hollows are where the purer limestone beds (calcium
carbonate) have been more easily dissolved than the more resistant ribs.
Small fossils and corals can often be found in the screes beneath the
outcrops. Picture taken from www.ruskinmuseum.com/geology.htm
Permian – Triassic
Since the formation of the Skiddaw Group in the Ordovician, the Lakes have slowly been drifting north,
starting from a position well south of the equator. During the late Devonian to early Carboniferous, it crossed the
equator and by about 250Ma it reached a latitude of the present day Sahara Desert. A landscape of dune sands and salt
kales developed, with seasonal downpours of rain washing debris from nearby uplands onto stony plains. These
conditions continued during the Permian and Triassic periods and ended about 190Ma.
These Permo-Triassic beds
show the same distribution
as the Carboniferous rocks
due to the doming of the
Lakes during the Variscan
Orogeny
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25km
Fig 4. This map shows the
distribution of Permian and
Triassic rocks shown in
red/brown.
Triassic – Present
Since the formation of the Triassic rocks, many more have been deposited in what is now the British Isles, but
none of them are now found in the Lake District. About 2Ma the Earth’s climate cooled and glaciers formed in
mountainous areas, including the Lakes, eventually spreading out over most of the rest of the country. During the
warmer spells the ice melted, only to return again when the climate cooled again. It is the action of glaciers and the
glacial melt waters of frost and tundra conditions which have shaped the surface details of the Lake District.
St John’s Vale Geology:
This granite intrusion is most probably part
of a much larger batholith, which is centred
underneath the Lake District. It was most
probably intruded in the Ordovician, when
the Caledonian orogenic belt formed. It
cannot be seen on this map; however
microgranite can be seen, which formed in at
an intermediate cooling rate, most likely near
the surface, but not penetrating it.
U-shaped valley
of St John’s Vale
N
The Skiddaw
Group of
Ordovician
Age
This is the Causey Pike fault. This caused
lateral displacement and trends in a NE –
SW. This was also probably a product of
the Caledonian Orogeny as it cross cuts
Ordovician beds; however it is difficult to
be certain as no other cross cutting
relationships are shown.
This U-shaped
valley was created
by glacial action of
what was likely to
be the last glacial
maximum 20,000
years ago in the
Holocene.
N
15 km
The area is heavily faulted due to the
immense amount of stress applied to
it. At least two orogenic events have
effected the Lake District and as a
result faulting and folding has
occurred (faulting shown by dark
linear structures).
Topography data is © Crown Copyright
Ordinance Survey. An EDINA Digimap / JISC
Supplied service
Geological data is © British Geological Survey,
Digital Geology Data
This is the Coniston fault. It
trends in a north – south direction
and was most likely a product of
the Caledonian Orogeny.
The Borrowdale Volcanic
Group (BVG) of
Ordovician age underlying
the majority of this area,
however sedimentary
successions have been
deposited on top, but
have been partly eroded
by glacial action.
Bibliography:
Steven Stanley: Earth System History 1999
Andrew McLeish: Geological Science 1986
www.ruskinmuseum.com/geology.htm