Aerial view from above Haystacks looking north-west over Buttermere and Crummock
Water. Ennerdale just visible top left.
Illustrated glossary of technical terms used in the book
Click on the terms coloured blue to open a new window with an accompanying photo or link
An episode of mountain building that took place between 375 and 325 million years ago (during the Devonian Period of geological time), also sometimes known as the Caledonian Orogeny. It predates the opening of the Atlantic Ocean, and produced an ancient mountain belt extending through what is now northern Europe, Scandinavia and the north-east of North America.
A group of volcanic ash beds (tuffs) in the Borrowdale Volcanic Group (see Excursion 3, Seathwaite)
A fan-shaped accumulation of silt, sand and gravel deposited by a river. Many fellside becks deposit alluvial fans when they reach the flatter ground of the valley bottom, and the fans frequently extend into lakes. The image shows the alluvial fan formed by Rannerdale Beck as it flows into Crummock Water.
A mineral produced when clay-rich rocks are heated to above around 500 degrees C while still in the upper part of the Earth’s crust, as close to a large body of igneous rock. Crystals are often strongly elongated and appear grey, white or pale pink. The image shows needle-shaped andalusite crystals in a Skiddaw Group hornfels. In rocks with graphite present, andalusite crystals incorporate inclusions of graphite, often forming a variety known as chiastolite. It is made of aluminium silicate . See also chiastolite, and Excursion 2.
A pale grey volcanic rock, with a silica content intermediate between basalt and rhyolite. Characteristically, andesites have a texture in which coarser white crystals of feldspar lie in a fine-grained matrix. Andesite lavas and sills make up much of the lower part of the Borrowdale volcanic sequence. Today, andesites erupt from volcanoes situated along plate margins such as the Andes - from where the name derives.
A sharp ridge formed where glacial erosion in adjacent cirques has cut back the intervening mountain. The image shows Sharp Edge, an arete on Helvellyn.
A grey iron-arsenic sulphide mineral, common in the mineral veins of the Coniston area (though not mined commercially).
The landmass lying to the south of the Iapetus Ocean in Ordovician times, and now underlying much of England and southeast Ireland. Mud and silt from rivers flowing from Avalonia into the Iapetus Ocean produced the thick sequence of rocks known as the Skiddaw Group.
(Official name: Bannisdale Formation). Marine mudstones and sandstones of Silurian age, belonging to the Windermere group. These rocks underlie large parts of the southern Lake District. The photo shows Bannisdale slates on Blawith Common, Coniston.
A very dense white mineral composed of barium sulphate. It is a common mineral in the waste tips of old lead mines, but is used in paint manufacture, and once worked commercially at Force Crag (near Braithwaite), and in the northern Caldbeck Fells. The baryte crystal shown in the photo is from Frizington in West Cumbria.
A dark grey volcanic rock, containing less silica than andesite. It erupts at higher temperatures than other lavas, and flows more readily. Basalt forms sills and lava flows in parts of the lower Borrowdale volcanic sequence. The image shows the surface of a modern basalt lava from Iceland. The tiny holes seen in the image (known as vesicles) were produced by gas bubbles forming in the lava whilst it was still liquid.
Basin
In geology, a large depression in the Earth’s crust where thick layers of sediment accumulate.
Layers formed when sediment, or volcanic ash, settles onto the surface (either underwater or on land). The image shows alternating silt and mudstone beds in Skiddaw Group sediments.
A stream channel that has cut down through soil and other superficial sediments to solid bedrock. The image shows the bedrock channel of Calston Beck, Hartsop.
A stone wall built to protect livestock from severe weather. The image shows a bield in Mosedale, near Loweswater.
A type of mica found in igneous and metamorphic rocks. In the image of a specimen of Skiddaw Granite, the tiny black specks are biotite mica. Larger mica crytals split into thin sheets with a very glossy appearance.
The name given to the andesite lava flows and sills which form the lowest part of the Borrowdale volcanic sequence, lying directly on older Skiddaw Group rocks. The image shows Birker Fell andesite lavas on Castlerigg Fell, Derwentwater.
A primitive clay furnace used in pre-industrial times to produce small quantities of iron from iron ore.
A copper-iron sulphide mineral common in the mineral veins of the Coniston area, it which tarnishes to bright metallic colours, giving rise to the name 'peacock ore'. It forms when veins of the primary copper ore chalcopyrite are weathered deep beneath the surface. It was much prized by miners because it has a higher copper content than chalcopyrite.
The formal name for the 6km thick sequence of Ordovician age volcanic rocks underlying the craggy fells of the central Lake District.
A glacial deposit of clay, sand, silt, and boulders. Boulder clays are common in valley bottoms, and in the lowlands surrounding the Lake District. Also known as ‘till’. The image shows a boulder clay in Mosedale, near Threlkeld.
A group of small marine shellfish with two shells (known as 'valves'). The valves are normally of different sizes, and are joined along a hinge-line. Brachiopods were one of the most important animals living on the sea-bed during Ordovician and Silurian times, and a few species still survive today.
Fossil brachiopods are common in some Windermere group sediments, particularly in the Dent Group (Coniston Limestone), The image shows a brachiopod shell in Dent Group limestone at Skellgill, near Ambleside.
A succession of dark grey marine mudstones and siltstones, part of the Windermere group. Brathay Flags have been used extensively as building stones in the Ambleside area.
A texture seen in rocks composed of broken fragments of older rocks. In the Borrowdale Volcanic Group, some breccias were formed by explosions in volcanic vents breaking up older volcanic deposits, or when the crust of a lava flow was broken up during the movement of molten lava (this is known as a flow breccia, and is illustrated in the photo of an andesite lava flow from the Eycott lavas). More commonly, brecciated texture is seen in sedimentary rock. A good example of a sedimentary breccia is the Permian Brockram seen in the Eden valley and at Saltom bay near Whitehaven. At Saltom the Brockram consists of a mass of angular and some rounded fragments of older rocks (Carboniferous limestone, Carboniferous sandstone and Borrowdale volcanics lying unconformably on Carboniferous (Westphalian) sandstones. See image here.
Bronze Age
The period of human history following the Neolithic. It started around 4400 years ago and ended around 2800 years ago.
CaCO3, calcium carbonate, the main constituent of limestone. The photo shows translucent calcite crystals from Frizington, West Cumbria. Calcite is more readily weathered than most minerals.
A depression in the Earth’s crust formed from the collapse of a magma chamber following an explosive volcanic eruption. The image shows the probable stages in the development of the Scafell Caldera in Ordovician times.
Carbon-14
A naturally occurring isotope of the element carbon, with a half-life of 5700 years. Carbon-14 is continually being generated as cosmic rays interact with nitrogen in the atmosphere. The proportion of carbon-14 to the stable isotope of carbon in organic material such as bone is used to measure its age. (see radiocarbon dating)
The geologic period that followed the Devonian and lasted from 360 to 299 million years ago. Carboniferous limestones and sandstones underlie large areas of the lowlands surrounding the Lake District, as well as the Pennine moorlands to the east. The photo shown Carboniferous Limestone countryside near Silverdale looking northeast towards Holme Park Hill.
A yellow copper-iron sulphide mineral, and the principal ore of copper. It has a more intense yellow colour than the more widespread mineral pyrite. See also, bornite.
A variety of the mineral andalusite, forming elongate white crystals. These crystals display a characteristic pattern of graphite inclusion which give them the appearance of a Maltese cross when viewed in cross-section. Heat from the Skiddaw granite caused the growth of chiastolite crystals in nearby Skiddaw Group mudstones (shown in photo).
A type of greenish-coloured clay mineral which commonly forms in igneous rocks as hot water flows through them during metamorphism. The greenish hue to many of the Borrowdale volcanic rocks is due to the presence of chlorite, as illustrated by the volcaniclastic slate from Honister (shown in photo).
A mountainside hollow produced by glacial erosion, and one of the most characteristic landforms of glaciated uplands. Many cirques in the Lake District are occupied by tarns.
Clay
A family of very fine-grained aluminium silicate minerals. Clays have the ability to absorb water and become plastic when wet. They are a common constituent of soils, glacial boulder-clay, and many sedimentary rocks.
found commonly in sedimentary rocks, soils, and glacial boulder-clay.
In geology, cleavage refers to parallel planes of weakness in rock, caused by alignment of platy minerals, such as micas, in response to intense deformation of the rock mass. The photo shows diagonal cleavage planes running through Skiddaw Group mudstone.
The blocks that form the surface of limestone pavement. C.f. gryke. The image shows clints and grykes on Whitbarrow Scar (see Excursion 7)
The name given to sedimentary rocks of the upper (younger) parts of the Carboniferous sequence in the UK that are rich in coal seams. In West Cumbria the Coal Measures outcrop between Maryport and Whitehaven. The map shows last century's principal coal mines in Cumbria.
A sedimentary rock composed of rounded pebbles derived from the erosion of older rocks. Beds of conglomerate are found in the Old Red Sandstone (Devonian) deposits at the northwest end of Ullswater, around Great and Little Mell Fell.
The old name for the lowest beds of the marine Windermere group, consisting of muddy limestones, lime-rich silts and muds of Ordovician age, often with abundant shelly fossils, and lying directly above the Borrowdale Volcanic Group. Now officially known as the Dent Group, they outcrop in a narrow SW-NE band between Coniston and Shap. The photo shows the Coniston Limestone at Tarn Hows (see Excursion 4).
A magnesium-iron silicate mineral produced by metamorphism of clay-rich sediments. Cordierite and andalusite crystals produce the 'spotted hornfels' seen in Skiddaw Group mudstones affected by thermal metamorphism close to the Skiddaw Granite. (See Excursion 2). Unlike andalusite, cordierite is readily broken down by weathering and so on a weathered hornfels surface, the andalusite grains stand proud while cordierite gives rise to depressions.
Cretaceous
The geological period between 145 and 60 million years ago. For the last part of this period, shallow seas advanced across much of England, including Cumbria, to give deposits of the limestone known as Chalk.
Thin layers of sediment inclined at an angle to the main horizontal bedding plane, and produced by ripples (underwater) or dunes (on land). Subaqueous ripple cross-bedding is seen in some Borrowdale Volcanic ash sediments, indicating that they were laid down in crater lakes (see Excursion 3). The photo shows cross-bedding (above the lens cap) in desert sandstones of Triassic age on the Solway Coast near Maryport.
The rocky outer layer of our planet. The crust of the continents is 30-50km in thickness, and the deeper parts are often very old. Below the deep oceans the crust is younger and much thinner.
A type of volcanic rock that is intermediate in silica concentration between andesite and rhyolite. In the Lake District dacite lavas are found in the younger parts of the Borrowdale Volcanic Group of the central fells. Dacite lavas are 'sticky' - they don't flow for any distance and typically build up as domes. Many of the volcanic ash deposits in the Lake District are also of dacite composition.
Dent Group
The modern official term for the Coniston Limestone.
Devensian
The name given to last Glacial Period, lasting from around 115, 000 to 11,500 years ago. Most glacial deposits in Cumbria date from this period, which obliterated deposits from earlier glaciations.
Devonian
The geological period that followed the Silurian, lasting from 420 – 359 million years ago. During this time most of England , apart from the southwest, was a land mass with basins in which sediments accumulated to form the Old Red Sandstone.
The prehistoric land area now submerged by the southern North Sea. During, and immediately after, the last Glacial Period, when sea levels were lower, Doggerland connected eastern England to the Continent.
A mound of glacial boulder-clay formed under a moving glacier. The photo shows a drumlin swarm at Whicham, near Silecroft, SW Cumbria.
In agriculture, the appropriation of common land by individual landowners. In the Lake District the most active period of enclosure lasted from about 1750 – 1860, and resulted in the long drystone walls that traverse the fells. The photo shows an enclosure wall in Mosedale, Wasdale.
The process by which loose material produced by weathering is moved from one location to another, usually by running water, the wind, moving ice, or simply the force of gravity. The photo shows erosion of boulder clay on the bank of Coledale Beck, near Braithwaite.
A boulder that has been moved by ice from the place where it originated and now rests on a different bedrock. The image shows an erratic boulder of Borrowdale volcanic andesite lava lying on Skiddaw Group mudstones, Broom Fell, near Wythop.
The belt of volcanic rocks that lie to the north of Skiddaw, from Eycott Hill in the east, to Binsey in the west are technically known as the Eycott Group. They consist of andesite lavas and sills, and are the same age as the lower parts of the Borrowdale Volcanic Group of the central fells. The photo shows a striking Eycott lava from Binsey with porphyritic texture.
A crustal fracture plane, along which movement has taken place. Faults vary in scale from a few centimetres to hundreds of kilometres in length. The photo shows small-scale faults revealed on a polished slab of volcaniclastic sandstone from the Borrowdale volcanics.
A silicate mineral common in igneous rocks, and often forming crystals large enough to see with the naked eye. The image shows pale feldspar crystals (known as plagioclase) in an andesite from the Borrowdale volcanics. Another form of feldspar
(known as orthoclase) is frequently pink in colour, and forms the attractive crystals seen in in the Shap granite.
Fell-dyke
A medieval wall (or embankment) separating the enclosed valley pasture from the rough common fell land. Also known as a head-dyke, or ring-garth.
In geology, refers to layers of rock that have been bent, distorted or buckled. Usually this happens deep in the Earth’s crust in response to tectonic forces, but folds can also sometimes form by slumping of wet sediments on the sea floor. Folds may be on the kilometre scale or just a few millimetres across, with small-scale folds often mimicking, or parasitic to, folds on a larger scale. The photo shows small-scale folding in Skiddaw Group rocks.
Freeze-thaw
The force applied when water in pores or cracks in rock freezes and turns to ice. Freeze-thaw action is one of the most important ways in which rocks in Lakeland are weathered.
A dark coloured igneous rock, similar in composition to basalt, but with a much coarser grain size. Gabbro forms an igneous intrusion on Carrock Fell. The image shows a lump of gabbro from this locality.
A dense grey sulphide mineral, the principal source of lead. Galena is particularly common in the mineral veins of the Skiddaw slates of the northern fells, where it was mined extensively from medieval times up to the early twentieth century. The photo shows a lump of galena from Force Crag mine, near Braithwaite.
Geochronology
Refers to a range of techniques that geologists use to measure the age of rocks. Radiometric dating is the most important tool in geochronology.
Glacial Period
An episode lasting thousands of years within a much longer Ice Age when average temperature on earth dropped back significantly, allowing glaciers to spread from the poles into lower latitudes. Between glacial periods are warmer interglacials such as the one we are living in now.
A large area of ice that does not melt in summer, and is steadily moving under its own weight. Individual glaciers can form in mountain valleys, but most of the Earth's glacial ice is in the form of ice caps covering mountain rangers or entire continents. Photo shows a mountain glacier on Blinnenhorn, European Alps. (Credit Louis Melahn, creative commons).
Glass
In geology, glass refers to a volcanic rock that has cooled so quickly that individual crystals do not have time to form, and the constituent atoms are not ordered into regular structures.
A medium- or coarse-grained igneous rock similar in chemical composition to rhyolite or andesite, but which has cooled and crystallized slowly within the Earth’s crust rather than being erupted as a lava. In the Lake District, the Eskdale and the Ennerdale granites, both of Ordovician age , are the most significant in terms of size. The smaller Shap and Skiddaw granites are of Devonian age. The photo shows the Shap granite displaying characteristic granite minerals: large pink orthoclase feldspar crystals, white plagioclase feldspar, grey quartz, and small black flakes of biotite mica.
A naturally occurring form of the element carbon. Very pure graphite was mined at Seathwaite until the nineteenth century. The photo shows a lump of Seathwaite graphite.
An extinct group of marine creatures which lived in branch-shaped floating colonies. Graptolites evolved rapidly through Ordovician and Silurian times, before becoming extinct. The range of fossil forms in successive beds makes them ideal for resolving the sequence of Skiddaw and Windermere group sediments. The photo shows Phyllograptus specimens in Skiddaw Group mudstone.
Grykes are the cracks between adjacent clints on limestone paving. The photo shows clints and grykes on limestone pavement, Whitbarrow Scar.
A soft calcium sulphate mineral found in the Permian rocks of Cumbria. Gypsum used to be mined at St Bees, where it was the basis for the local chemical industry. It is still mined in the Eden Valley, near Penrith, for uses such as plasterboard.
Half-life
In physics, the time it takes for half the atoms in a sample of a radioactive isotope to decay. The concept of half-life is fundamental to the use of radioactive isotopes in geochronology.
A subsidiary valley above a main valley and separated from it by a steep slope. Hanging valleys are a classic feature of glaciated mountain landscapes, and are common in Lakeland, particularly in the high central fells. The photo shows the valley of Watendlath, which hangs over Derwentwater.
A red iron oxide mineral, the chief source of iron. Vast quantities of hematite were mined in West Cumbria in the 19th and early 20th century, from ore deposits found mainly in Carboniferous limestone. The photo shows Hematite from Knockmurton, near Lamplugh.
An attractive blue-green volcanic ash from the lower part of the Borrowdale Volcanic Group, still quarried today on the sides of Fleetwith Pike near Buttermere.
A hard rock, very difficult to split, produced when mudstone is metamorphosed by heat from an igneous intrusion. In the Lake District hornfels is found in Skiddaw slate sediments close to the Skiddaw Granite (shown in photo), and around the (hidden) granite intrusion near Crummock Water.
The ocean basin that, in Ordovician and Silurian times, lay between Laurentia (to the north) and Avalonia (to the south). Skiddaw Group and Windermere group marine sediments were deposited on the floor of the Iaptetus.
Ice Age
A long period of earth history during which the planet repeatedly oscillates between glacial (cold) and interglacial (warm) climatic regimes. The current Ice Age started about 2.5 million years ago.
Igneous
A rock that started life in a molten state, such as a granite or a volcanic rock.
A body of igneous rock that cooled and crystallized from molten state before reaching the earth’s surface. The insulation provided by the overlying rocks means that heat is lost slowly and crystals grow much larger than in equivalent volcanic rocks. The image shows the contact between a granite (below key) intruded into grey Bannisdale slate (Brown Howe quarry, Torver).
A volcanic rock formed from fast-moving currents of ash, lava drops and hot gas that can occur during explosive volcanic eruptions. Ignimbrites are common in the sequence of volcanic rocks associated with the Scafell and Langdale caldera eruptions. The photo shows an ignimbrite from Crinkle Crags. The dark wavy areas are flattened fragments of lava which were originally spherical, but, because they were still soft and molten on deposition, were squeezed flat by the weight of the material settling above them.
The improved valley pastures, usually separated from the rough outbye grazing of the fellsides by a stone wall running around the valley sides (the fell-dyke). The photo shows the contrast between bright green in-bye pasture, and rough fell grazing on Maiden Moor, Newlands.
Fellside land which has been ‘taken in’ by farmers and added to the good quality pasture of the valley bottom by enclosure and improvement. The photo shows intack fields at Gill Bank farm Eskdale.
Iron Age
The period following the Bronze Age. In northern England very roughly from 800 BC to 100 AD.
Isotope
Isotopes are alternative forms of the same element distinguished from one another by having different number of neutrons in their atomic nuclei.
A simple fracture in rock caused by stress, and not involving any regrowth or reorientation of the rock’s mineral grains (cf cleavage), or displacement (cf fault). Joints often form parallel sets of cracks running through a rock outcrop. The photo shows joints in the Eskdale granite.
A clay mineral formed from the chemical weathering of feldspar. The photo shows iron-stained peaty kaolinite near Sinen Gill, the result of weathering of feldspar in the Skiddaw granite.
A type of landscape found in limestone areas, with features such as clints and grykes caused by dissolution of limestone by rainwater. The photo shows karst on Whitbarrow Scar.
A hummocky landscape of rocky hillocks and intervening hollows, produced by glacial scouring. The classic knock-and-lochan is found in northwest Scotland, form where the term originates, but in the Lake district it is common on volcanic and granite terrains between the main summits and ridges. The photo shows knock and lochan granite scenery in Upper Eskdale.
A slurry of mud and rock fragments flowing down the flanks of a volcano, triggered by eruption, rainfall, or a bursting crater lake. Some deposits in the Borrowdale volcanic sequence have been interpreted as lahars.
Molten rock (magma) that reaches the Earth’s surface.
A rock composed largely of calcium carbonate originating from the remains of marine organisms. In Cumbria limestones of Carboniferous age are common around the peripheries of the Lake District. Impure limestone beds also occur in the Dent Group (Coniston Limestone). The photo shows beds of Carboniferous Limestone near Moota, north of Cockermouth.
A feature of karst landscape characterized by flat-lying areas of limestone blocks (clints) incised by fissures grykes. The photo shows limestone pavement near Moota, north of Cockermouth.
A simple rectangular building. The remains of medieval stone-built longhouses have been identified in the Lake District, some may be Viking. The photo shows the remains of a double-walled medieval longhouse in Ennerdale.
Part of the Skiddaw Group sedimentary sequence, characterized by alternating beds of mudstone and fine-grained sandstone, seen principally in the fells around Loweswater and Lorton in the northwest Lake District.
Molten rock. All igneous rocks originate as magma.
A reservoir of magma in the earth’s crust. Volcanoes are fed from magma chambers.
(Botanical name Nardus stricta) a wiry grass common in overgrazed uplands in the Lake District. The photo shows mat-grass on Great Moss.
Mesolithic
The period of human history preceding the Neolithic and lasting in Cumbria between about 11500 and 6000 years ago.
Metamorphism
The process by which rocks are altered from their original state by heat, pressure and/or the circulation of hot water within the Earth’s crust.
A band of distinct coarse-grained minerals running through a rock mass, in which ore minerals occur. Mineral veins may cut through a uniform rock mass or may occur in a fault zone. are concentrated. The photo shows a mineral vein in Coppermines Valley, Coniston.
A mound or ridge of boulder clay left by a retreating glacier, or deposited around the margins of an active glacier. Many Lakeland valleys contain moraines of varying sizes and shapes, thought to have been formed during the Younger Dryas, the last period of active glaciation in the region. The photo shows hummocky moraines in the upper Wythburn valley.
A rsedimentary ock formed from very fine-grained particles, mostly clays. The photo shows folded Skiddaw Group mudstones.
Neolithic
The period of human history following the Mesolithic. In northern England between about 6000 and 4400 years ago.
A mountain peak rising above an ice cap.
Old Red Sandstone
A group of sedimentary rocks including sandstones, siltstones and conglomerates deposited during the Devonian Period (420-359 million years ago). In Cumbria, Old Red Sandstone rocks are found only in a limited area around Pooley Bridge.
Ordovician
The period of Earth history lasting from about 485-443 million years ago. Rocks of the Skiddaw Group, The Borrowdale Volcanic Group and the lowest parts of the Windermere group were all laid down during this period. The Eskdale and Ennerdale granites are also of Ordovician age.
Orogeny (or orogenesis)
The processes in the Earth’s crust by which mountain belts are created. Orogeny involves the coming together of originally separate areas of crust to form a belt of deformed and metamorphosed rocks which afterwards acts as a single tectonic plate.
Outbye (or out-by)
Rough unenclosed fell pasture used for summer grazing (cf Inbye).
The flat area covered in sand and silt carried by meltwater issuing from the end (terminus) of a glacier.
A mass of partly decomposed plant debris, often waterlogged.
A rock with an unusual texture formed when hot magma interacts with waterlogged sediment. Lava intruded into the sediment is quenched and forms small glassy blocks, isolated in the enveloping sediment. Peperites are very characteristic of parts of the lower Borrowdale volcanic sequence (around Honister for example, as shown in photo), where andesite sills and lavas came into contact with lake sediments.
Permian
The period of earth history following the Carboniferous and preceding the Triassic. It lasted from about 299 to 252 million years ago. The gypsum deposits in the Eden valley are of Permian age.
Pigmyweed (or New Zealand Pigmyweed)
A highly invasive aquatic plant imported to the UK from New Zealand early in the last Century, and now a serious threat to the ecology of many Cumbrian lakes.
Pollen analysis
The study of plant pollen preserved in peat bogs or lake sediments. Combined with radiocarbon dating it is an important tool in reconstructing ancient environments and environmental change.
A texture seen in some igneous rocks where relatively large crystals of a mineral (usually feldspar) lie in a finer-grained matrix. The Shap granite , and some andesites from the Borrowdale Volcanic Group display this texture. The photo shows porphyritic andesite from the Eycott lavas on Binsey, near Bassenthwaite.
A yellow copper/iron sulphide mineral, also known as ‘fools gold’. Pyrite is common in Lake District mineral veins, and as scattered grains in Skiddaw slates and Brathay Flags. The photo shows 2mm wide bronze-yellow pyrite crystals on calcite from Frizington, West Cumbria.
Pyroclastic rock
A rock formed during an explosive volcanic eruption, containing ash from the explosive fragmentation of magma and/or previously solid rock. Tuffs and ignimbrites are examples of pyroclastic rocks.
Also known as a pyroclastic flows or nuées ardentees, these are extremely hot and destructive clouds of pyroclastic material, steam and air which surge down the flanks of volcanoes during explosive eruptions (see also ignimbrite). The photo shows a pyroclastic density current rushing down the flanks of Mt Merapi, a volcano in Indonesia.
A form of silicon dioxide (SiO2), quartz is one of the most abundant minerals in the Earth’s crust and is notably unaffected by chemical weathering. It is a major constituent of many sandstones and siltstones, is abundant in some igneous rocks (particularly granites), and also produced by metamorphism. It is also the commonest vein mineral. The photo shows white quartz veins permeating grey Skiddaw Group mudstone.
Radiocarbon dating
The use of the carbon-14 isotope to determine the age of organic material. The short half-life of carbon-14 means that radiocarbon dating is most useful for material up to about 50,000 years old.
Radiometric dating
The measurement of isotopes to determine the age of a rock or mineral. A number of elements in that occur naturally in minerals have radioactive isotopes. These isotopes break down at a constant, known rate to produce a distinctive daughter isotope of a different element. The principle of radiometric dating is to measure the amount of the radioactive isotope remaining today and the amount of daughter isotope produced since the mineral grew. Radiometric dating has enabled geologists to obtain ages for most of the key geological events in Earth history. Modern technology means that radiometric ages can often be measured to within 2% of the true value.
An elongated water body occupying a glacial trough. The photo shows Wastwater, a classic ribbon lake.
Ring-garth
Walls built by Viking settlers to surround the valley bottom grazing. Similar in function to later medieval fell-dykes.
A rock outcrop eroded by moving ice to produce an asymmetric profile, with a smooth, rounded ‘upstream’ side and a steeper craggy ‘downstream ‘ side. The photo shows a roche moutonnee (Barber's Rock) near Crummock Water.
A type of landslide involving the failure and collapse of a section of hillside. In the Lake District there is evidence of at least 50 such events following the retreat of glaciers. The photo shows the large Threlkeld Knotts rock slope failure.
A sedimentary rock composed of sand-sized grains, defined as 0.0624 – 2mm. These are often dominated by quartz or feldspar, but many other minerals and rock fragments can also occur. Sandstone beds are common the parts of the Skiddaw Group and Windermere group rocks of the Lake District, and in Carboniferous and Permo-Triassic sediments of the surrounding lowlands, while volcaniclastic sandstones occur in the Borrowdale Volcanic Group. The photo shows Triassic sandstone near Whitehaven.
A sequence of volcaniclastic sandstones in the upper Borrowdale Volcanics. They have been an important source of blue-green slates. The photo shows a slab of Seathwaite Fell slate from a quarry on Coniston Old Man.
Sediment
In geology, sediment is any loose material (such as sand grains, shell fragments, or mud particles) deposited on the earth’s surface.
Sedimentary rock
A rock formed when sediment is consolidated as it becomes buried. The weight of overlying deposits compacts the sediment but chemical processes are also important in turning it to rock, with minerals such as calcite or quartz being precipitated in the pores of the sediment to act as an effective glue. Sandstone, mudstone and limestone are all sedimentary rocks.
A shepherd’s shelter or dwelling, used in the summer months when flocks were taken to the uplands. The photo shows a ruined medieval shieling on Whiteless Breast, above Buttermere.
Silica
Silicon dioxide (SiO2), is an important constituent of many rocks and minerals. Quartz is a mineral composed only of silica.
In geology, a body of igneous rock that has been intruded between preexisting layers of rock, to form a parallel layer. Sills are common in the Borrowdale Volcanic sequence. The photo shows stacked sill sheets on Fleewith Pike, Buttermere.
A sedimentary rock made up of silt-sized particles, smaller than sand but larger than the clay particles which dominate mudstones. The photo shows thin alternating bands of siltstone (grey) and mudstone (brown) in the Skiddaw Group.
Silurian
The period of Earth History between about 443 and 420 million years ago, after the Ordovician, and before the Devonian. Most of the Windermere group rocks of southern Lakeland are Silurian in age.
A depression caused by the collapse of the surface layer of rock. Sinkholes are common on the Carboniferous Limestone country of Cumbria and result from the underground dissolution of limestone by rainwater, and subsequent collapse the cavern roof.
Also known as Skiddaw Slates, these marine mudstones and sandstones were deposited in Ordovician times, and folded and weakly metamorphosed during the Acadian Orogeny. They are the oldest rocks in the Lake District, and underlie the northern fells. The photo shows Skiddaw Group country - Gasgale Gill, Crummock Water, and Melbreak.
A deformed and metamorphosed mudstone that splits along pervasive parallel surfaces (cleavage planes). These are the result of regrowth of clay minerals perpendicular to the compressive stress that drove the deformation. The photo shows a quarry at Coniston with near vertical cleavage planes running through chlorite rich Borrowdale Volcanic Group rock.
Gradual downslope creep of soil and other unconsolidated material. The photo shows solifluction deposits lying above glacial boulder clay, Coledale near Brathwaite.
Zinc sulphide, a common ore mineral in Skiddaw Group mineral veins of the northern fells. Also known as zinc blende, it is brown in colour and lacks the metallic appearance of most sulphide minerals. The photo shows sphalerite crystals from Force Crag mine, Braithwaite.
Officially the Stockdale Group, these marine mudstones and siltstones belong to the Windermere Supergroup. They lie above the Dent Group. The photo shows Stockdale shales at Tarn Hows.
Scratches on a rock surface caused by glacial abrasion. Photo shows striations on Borrowdale volcanic tuff, Church Beck, Coniston.
A channel cut by meltwater flowing under a glacier. Some Lake District ravines like Ruddy Gill below Great End (shown in photo) may have formed from enhanced bedrock erosion along subglacial channels.
A small body of standing water. Mountain tarns often occupy cirques on fellsides, or other glacially eroded depressions. The photo shows Bowscale Tarn, the most northerly cirque tarn in Cumbria.
(Also known as contact metamorphism). The mineralogical changes that take place in a rock when it is heated by intrusion of a body of magma. The thermal metamorphism of the Skiddaw slate close to the Skiddaw granite is a good example (see Excursion 2). Here, the growth of new randomly oriented mineral grains changes slate to hornfels.
The fossilized tracks, burrows or other impressions left in a sedimentary rock by the activity of extinct animals, rather than the remains of the animals themselves. Trace fossils are common in parts of the Skiddaw Group, as shown in photo.
A step-like landform resulting from the alternation of hard erosion-resistant lava flows or sills with intervening softer layers. In the Lake District it is seen in stacked andesite lava flows and sills in the lower parts of the Borrowdale Volcanic Group, particularly at Eycott Hill (see Excursion 1), and High Rigg near Keswick (shown in photo).
Triassic
The period of geological history that followed the Permian, from about 252 to 201 million years ago. Most of the red sandstones of the Eden Valley and West Cumbria are Triassic in age.
An extinct group of arthropods. The accompanying image shows a trilobite from marine sediments of the Silurian Windermere group (actual size approx 2 cm in length).
Rock formed from hardened volcanic as, ranging in grain size from mud to sand. The photo shows a tuff from near Po House farm near Millom. Each band represents a separate episode of ash fall from a nearby eruption.
A submarine avalanche of sand and mud. Beds deposited by turbidity currents are common in parts of the Skiddaw Group and Windermere group sequences. The photo shows a turbidite sandstone bed from the Silurian Windermere group on Gawthwaite Moor, Furness.
Uranium isotopes
Varieties of the element uranium distinguished from one another by having different numbers of neutrons in their atomic nuclei. The most abundant isotopes - uranium-235 and uranium-238 - decay to isotopes of lead and are used in radiometric dating.
The characteristic cross-section profile of a glaciated valley. The photo shows the U-shaped Newlands Valley, as seen from Robinson.
A sedimentary rock formed when the products of volcanic eruptions (lava, ash etc) are eroded and redeposited, often shortly after their original deposition. Many of the Borrowdale Volcanic Group rocks are volcaniclastic sandstones deposited in caldera lakes. The photo shows a volcaniclastic sandstone from the Seathwaite Fell Formation.
A drystone enclosure next to a beck where sheep were washed. Washfolds pre-date the era of chemical dipping. The photo shows a large well-preserved washfold in the Wythburn valley near Thirlmere.
The process by which rock is broken down by physical, chemical, or biological processes.
Weathering refers only to the processes that break down the rock: removal of the products of weathering is termed erosion. The dissolution of limestone by acid rainwater is an example of chemical weathering, illustrated in the accompanying photo of limestone pavement on Whitbarrow Scar. In contrast, feldspar in granite is weathered to clay, so that the granite erodes to give rounded hills.
A bed of tuff widespread in the central fells. It lies directly above Birker Fell andesite, and marks the start of the explosive phase of the Borrowdale volcanic sequence. The photo shows the Whorneyside Tuff near Sour Milk Gill, Gillercomb.
(Officially known as the Windermere Supergroup). The thick sequence of marine sediments (of end-Ordovician and Silurian age) that lie above the Borrowdale Volcanic Group. These sediments underlie much of the southern Lake District. The photo shows typical Windermere group scenery around the southern end of Coniston Water, contrasting with the rugged Coniston fells (Borrowdale Volcanic Group) on the skyline.
A black iron-manganese-tungsten mineral, the principal ore of tungsten and once mined at Carrock Fell. (See on-line excursion to Mosedale).
Younger Dryas
(Also known as the Loch Lomond Re-advance). The period of arctic conditions that returned to northern Europe between 13000 and 11500 years ago, after the end of the main Glacial Period.
A silicate of the element zirconium. Zircon crytals can remain unaltered by geological processes for hundreds of millions of years and the uranium and lead isotopes they contain are used in radiometric dating.