Geology of the Lake District
Ordovician
Igneous Intrusions
Jurassic
Figure 1
Scale 1cm: 1km
(all Fig.’s are at
front of view)
Carboniferous
Unconformity
Carboniferous
Permo-Triassic
The scenery of the Lake District was created largely by glacial movements of the last ice age between
25,000 and 10,000 years ago and the volcanic eruptions that preceded it, however the geology has a
much longer history. During the last Ice Age, the area was the source of many glaciers which carved
the deep U-shaped valleys and sculpted the sharp ridges or arêtes characteristic of the area. When
the ice finally retreated about 10,000 years ago, rivers formed and created exacerbated valley
features which lakes later in-filled, giving the scenery that we see today. However, 10,000 years is
very recent in terms of geological history. The recent glaciers re-worked rocks as old as the
Borrowdale Volcanic series, which formed about 500 million years ago, during the Ordovician.
In the north, the region of Skiddaw Slate is a landscape characterized by relatively smooth outlines
and although often impressively high (Skiddaw 3,053 ft; Blencathra 2,847 ft) hills lack the craggy,
rugged features to be found in the central Borrowdale Volcanic region (Scafell Pike 3,210 ft). Most of
the high peaks are composed of these igneous rocks which have been able to withstand erosion well.
The first rocks deposited in this area were the Skiddaw Slate Series which was formed due to
intense compression in the early Ordovician. The base of this series is unknown; therefore it could
amount to several thousand feet of sediment. These sediments were formed about 500 mya by
deposition in a shallow sea, low energy environment. The series differs in grain size and comprises
grits, flags, shales and mudstones. The Skiddaw Granite has intruded into the Skiddaw Slates close to
the anticlinal axis of the Lake District. There are three outcrops; Sinen Gill, in the Caldew valley and
near the junction of Caldew and Grainsgill. The Carrock Fell intrusion is to the north of the Skiddaw
Granite and is a whole pluton complex which is parallel to the grain of the country rock of the
Borrowdale Volcanics. The northern
and eastern boundaries of this
intrusion are faulted.
The next rock sequence in this area is
the Borrowdale Volcanic Series
which is thought to total 10,000 ft in
thickness. Due to the thickness of the
series
and
its
resistance
to
weathering, the majority of the Lake
Districts highest peaks are composed
of these igneous rocks (Scafell Pike
Scale 1cm: 600m
Figure 2
3,210 ft; Helvellyn 3,118 ft). The
Borrowdale
Volcanic
Series
is
composed of lavas (mainly andesites), tuffs and agglomerates, along with some major igneous
intrusions (shown as red in Figures). The sites of the volcanoes are unknown, but their activity is
thought to be periodic with marine deposition higher in the sequence. The contact between these two
Ordovician series varies along the suture but it tends to be a fault or a thrust, or at other places an
unconformity (shown as blue dashed line in Figure 2). The shales of the Skiddaw Slates overlie the
Borrowdale Volcanics creating a local unconformity because they successively overstep the volcanic
rocks.
There is a wide variation in composition, within the series which diverges over the area giving rise to
the mottled (blue and grey) effect shown in Fig. 1. Northern outcrops of this series exhibit a simple
structure, whereas those in the south show folding events. It is likely that a period of slight folding
occurred which was followed by erosion and then by the deposition of the Lower Silurian rocks.
The Silurian Windermere Group have a base of Coniston Limestone which are a group of
calcareous sediments which rest unconformably on top of the Borrowdale Volcanic Series shown by
the yellow dashed line in Fig 3 & 4. This is then succeeded by a series of shales and then by grits and
greywackes. Compression from the NW or SE buckled the strata into anticlines and synclines and
caused slaty cleavage in some sediment beds.
The Devonian was a time of alteration of previously formed rocks as little new rocks were formed.
After folding and faulting events, there was a period of major intrusions ranging from acidic to basic,
forming stocks to laccoliths to dykes. The most important of these is the Shap Granite (a laccolith
made of porphyritic biotite granites) shown in Figure 3, which was intruded into the Borrowdale
Volcanic Series near the junction with the Coniston Limestone. The largest intrusion in the Lake
District is the Eskdale Granite. There are two outcrops, of which the larger one covers Wastwater
across Eskdale to Bootle and the smaller one is confined to Wasdale. At the foot of Wastwater the
granite outcrop approaches the Ennerdale Granophyre, however contacts are not visible.
Carboniferous sediments rim
the
pre-existing
rock
successions
with
predominantly conglomerates
and limestone which are only
gently folded or horizontal.
There
is
a
definite
unconformity between these
younger
beds
and
the
Ordovician and Silurian beds
to the south, signifying a time
gap of around 100 my
Scale 1cm:300m
indicating period of erosion or
a gap in deposition. This
unconformity is illustrated in Figure 2 below, showing the steeply dipping folded beds in the south and
the younger horizontal beds to the north. The origin of these conglomerates is unknown due to the
unusual grain composition and they are composed mainly in this area of Dinantian sediments.
Figure 3
Shap
Granite
Intrusion
The Permo-Triassic sediments are largely composed of ‘millet seed’ sandstones indicative of a desert
environment. These sandstones are interbedded with breccias, containing included fragments of
Carboniferous rock and fine grained shales.
The Jurassic Lias sediments are difficult to observe but they are present in a small quantity, only as
an outlier in the north of the Lake District. They consist of dark shales with bands of bluish
argillaceous limestone. These sediments mark a widespread submergence at the end of the Triassic.
Later formations were probably deposited but were removed by denudation following major tectonic
processes in the Tertiary. These Tertiary processes raised the rock sequences into a dome from which
newer sediment was eroded and so left rings of Devonian and Carboniferous around a core of folded
Ordovician rocks. Erosion and glaciation has produced the landscape we now see today.
Folded Ordovician Rocks
Horizontal Carboniferous Rocks
Figure 4
Scale 1cm: 300m
Lower Palaeozoic
Unconformity
Carboniferous
Unconformity
St John’s Vale is of particular interest to the geology of the area. St John’s Vale is located in the
middle of two microgranite plutons, Low Rigg (GR. 302227) and Thelkeld (GR. 325221). These form
an uplifted topography that seems to be suffering from erosion on the eastern slopes. These two
plutons have intruded the Buttermere Formation, which consists of sheared and folded mudstone,
silts and sandstones.
The pale pink geology on the diagram above is called the Birker Fell Formation (Ordovician) and
consists of phyric andesite lavas and subordinate sills. There are also dacite lavas and interbeds of
tuff, pyroclastic breccia, conglomerate and volcaniclastic sandstone. This is part of the Borrowdale
Volcanic series that occupies the area south of Keswick and south-east of Buttermere.
Buttermere Formation
Figure 5
Micro-granite Plutons
St John’s Vale
Scale 1cm:200m
Birker Fell Formation
The faulting that is of most relevance to St. John’s in the Vale is the Coniston Fault Zone. The
Coniston Fault is easily recognisable by the unconformity that it creates. During the course of faulting,
the Brathay Formation and Borrowdale Volcanic Group have been brought into contact. Along the fault
line crushed grey siltstone is in sharp contrast with grey-green brecciated and silicified volcanic rocks.
The boundary itself is marked by a rib of the volcanic rock. The fault shows apparently dextral offset.
The exact trace of the fault can be seen in Figure 6.
The Brathay fault is
a
sinistrally
displaced fault with
some
areas
that
exhibit
dip-slip
displacement
throwing down to
the East. The fault
has
produced
considerable
displacement of up
to 1.7km. This has
offset many of the
stratigraphic
sequences
by
a
great
distance
particularly in the
Windermere Group
and the Borrowdale
Volcanic Group. The
fault
is
clearly
Scale 1cm:500m
Figure 6
marked
at
the
surface by a major
fault scarp on its
western side. Brathay fault. This is a sinistrally displaced fault with some areas that exhibit dip-slip
displacement throwing down to the East. The fault has produced considerable displacement of up to
1.7km. This has offset many of the stratigraphic sequences by a great distance particularly in the
Windermere Group and the Borrowdale Volcanic Group. The fault is clearly marked at the surface by a
major fault scarp on its western side.
St. John’s
Vale
Figure 7
Scale 1cm:4km
Major faults trending
NNW-SSE and E-W
(shown in yellow)
N.B Ariel
Photograph
To the west of the Coniston Fault Zone is the Parks Gill Thrust. The fault is roughly strike-parallel and
shallowly dips to the south-east. The faults have been identified as a thrust fault by the offsets of the
stratigraphic sequence that it creates. The fault is marked by an easily identifiable gulley.
Parallel to the Coniston to, lies the Bowmanstead Fault. Unlike most of the other major faults, it shows
dextral displacement of an average of 50m in the Upper Wenlock Group.
The Banks Fault, like the Bowmanstead Fault, exhibits dextral displacement. It is a North trending
fault which displaces the Wenlock Group by up to 100m.
The Klondike fault zone comprises of two major faults and a large system of smaller fractures. The
Klondike fault itself marks the eastern edge of the 2.5km wide zone. It is a north-north-east striking
fault with sinistral displacement (i.e. the apparent displacement is to the left). The fault is surrounded
by many smaller faults, which form a braided en-echelon system. This term refers to the fact that the
faults are arranged parallel to each other with oblique displacement. All the smaller faults show the
same sinistral displacement as the major fault
To the East of the Lake District lie two major north-north-east striking faults, the Wharton Tarn and
the Tarn Hows faults. They both produce an apparent sinistral offset of 120m in the Wenlock Group.
The Tarn Hows Fault can be clearly seen by the steep sided valley that runs south-south-west along
the fault.
References:
Kneller, B.C., 1990. BGS Technical Report: Wenlock Rocks (Lake District)
Kneller, B.C & Scott, R.W., 1990. BGS Technical Report: Ashgill and Llandovery age rocks (Lake
District)
Eastwood, T., 1946. British Regional Geology: Northern England, Second Edition.
Woodward, H., 1887. The Geology of England and Wales, Second Edition.
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supplied service; Geological data is © British Geological Survey, Digital Geology Data.