Field Studies:
Lake District Digital Geology Project
The landscape and the scenery of the Lake District rivals the most splendid scenes in
the United Kingdom. It boasts the highest mountains in England as well as containing
most of the countries lakes. Whereas most of the current topography was fashion
approximately 12,000 when the whole area was covered by a glacier, the underlying
bed rock is just as important when trying to understand the features of the Lake
District.
The oldest group of rocks in the Lake District were formed during the Ordovician
period and is name the Skiddaw group. It was formed around 500 million years ago
when mud settled on the sea bed in relatively deep water where other layers of coarser
silt and sand were then deposited on top. This group of rocks then formed a triangular
mountainous area in the region of Kewsick which are generally smooth in outline due
to thermal metamorphism that occurred in the area. 50 million years later, the district
entered a phase of cataclysmic volcanism which formed the Borrowdale Volcanic
Group, which formed rocks which are volcanic lavas and ash flows. Some minerals,
especially copper was formed in these volcanic eruptions which occurred in two
parts- a preliminary eruption of lava flows with an occasional explosive eruption and
then secondly a phase of much higher energy eruptions. This phase was caused when
the Iapetus Ocean was subducted beneath the continent to the south (the European
Plate) which the present day England is formed. These lavas are mainly Andesites,
with the occasional Basaltic and Rhyolitic rocks also found and are around 8km thick.
These are the rocks that are responsible for the highest parts of the Lake District such
as Scaefell Pike at 977m high. These rocks are found to the south-east of the Skiddaw
group rocks in the area of Grasmere, in central Lakeland.
This geological map shows the area around Keswick which is on the edge of an
igneous intrusion. It also shows some regions of Ordovician rocks which were laid
down around 500 million years ago.
The next rocks to form the Windermere group during the Silurian period about 420
million years ago. These rocks were mainly sedimentary slates, siltstones and
sandstones, greywackes and deep water shales also formed when this region was
below water. These rocks are not as resistant to the elements as the rocks that formed
during the Ordovician period and therefore do not formed high peaks. These rocks
spread down to the south including the towns of Coniston, Kendal and Windermere,
in a retroarc foreland basin.
Around 20 million years after these rocks were deposited, they were all faulted and
folded and then push upwards to form a high mountain range by the intrusion of
molten magma. This also caused the beds to be tilted at 90o in a south easterly
direction. This event was known as the Caledonian Orogeny and was caused when
two continents collided to form the new Eur-American continent and formed a
mountain belt that looked a lot like the Himalayas do today. This process also
compressed many of the Borrowdale Volcanic Group rocks to make them into slates
known as the Westmorland Green slates. These mountains have now been eroded
down to their present size and so the faults and igneous intrusions can now be seen
and are exposed in some areas of the Lake District. These all form part of a very large
granite ‘batholith’ which has uplifted the whole area. Most of the structure of the
Lake District is formed by a dome over the Ordovician-Silurian rocks, with the
change from these rocks being marked by a distinctive change in slope.
The geology shown above is taken from the west side of the lake district. It highlights
one of the major igneous intrusions and shows that the area has been exposed to sever
faulting and folding.
Around 350 million years ago most of the mountains were eroded away and most of
the land sank beneath a tropical sea. This period was called the Devonian period. The
sea floor then became covered with a thick layer of sediment, which consisted of
broken remains of shells and animal life which formed pale grey carboniferous
limestone. As a result, the base of the carboniferous shows a marked unconformity.
This makes these rocks interested due to fossils of snails occasionally being found in
the limestone. Following the deposition of these rocks, the Namurian Millstone Grit
was deposited along with coal measures, as the continent moved northwards. 70
million years later, these carboniferous rocks were then uplifted and folded in the
second orogeny, called either the Variscan or Hercynian orogeny, and continental
deposition of breccias and fluvial and Aeolian sandstones occurred.
This diagram shows the main unconformity in the area which is between the
Ordovician, Borrowdale Volcanic Group and the Carboniferouis, Westphalian layer.
During the Permian and Triassic periods, the landscape of sand dune and salt lakes
were formed as the deposition of dolomites and evaporates occurred, with the
seasonal downpour of rain which washed rock debris from nearby uplands. During the
Cretaceous and Tertiary periods, there is no evidence of any deposition in this area.
Around two million years ago, the Lake District was covered by a glacier when the
temperature of the earth fell and we went into an ice age. This shaped a great deal of
the topography of the lake district as the bulldozering effect of the glacier created
huge u-shaped valleys and also left other landforms that are typical of a glaciation
period. This also resulted in the deposition of glacial till (boulder clay), which may be
rounded later form heaps know as drumlins, and fluvio-glacial sands and gravels.
Larger heaps that formed at the melting snout of the glacier are also present and are
called terminal moraines.
References:
Bennison G.M, Wright A. E, ‘The Geological History Of the British Isles’ 1978
(London)
Smith, R.A, ‘A bibliography of Lake District geology and geomorphology’, 1952
(Whitehaven)
www.lake-district.org.uk
Topography data is ©Crown Copyright, Ordinance Survey, An EDINA Digimap/JISC
Supplied service
Geological Data is ©British Geological Survey, Digital Geology data.