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find out more about ourrich and varied islands

geos

weathered sandstone

cliff scenery

cove

r im

ages

: to

p f

oss

il f

ish

b

ott

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sea a

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Orkney’snaturalheritageOrkney’snaturalheritage

Orkney’snaturalheritageOrkney’snaturalheritage

Orkney’sGeologyGeology

Europe and ScotlandMaking it work together

Where there are now seventy or so islands inOrkney, dotted over an area of sea that liesbetween the North Sea and the Atlantic, thereused to be a great freshwater lake. This once vastlake existed nearly four hundred million yearsago, during the Devonian geological period, andstretched from what we now know as the MorayFirth, across Caithness to Orkney, Shetland andbeyond to the Norwegian coast. Given the name‘Lake Orcadie’ by geologists, this huge lake wasringed by high mountains, the ancestors of theGrampian and Northern Highlands. The mountainswere drained by mighty river systems thattransported enormous amounts of sediment intothe low lying areas. Sand and other loose sedimentthat did not reach as far as the lake, was blownaround by the wind to form sand dunes that wouldhave migrated across what was a hot, arid, desert-like landscape.

These layers represent the sediment that waswashed into Lake Orcadie from the surroundingmountains. It is thought that muds and sands toa depth of 4,000 metres were deposited and piledup in the lake and its surroundings. As timepassed and as environmental conditions changedover thousands and millions of years, the lakelevel fluctuated and areas which were previouslydry land were engulfed by water. At other times,the lake almost dried up completely.

in the crust of Devonian Scotland, induced bycontinental drift, sometimes gave rise to volcanicactivity, with the eruption of lava and ash overthe landscape from volcanoes. It was a result ofcontinental drift over the subsequent few hundredmillion years, that brought about Scotland’snorthward drift to its present latitude and thesplit between Britain and America with theformation of the Atlantic Ocean and North Sea.

Where was Orkney within this ancient environment?What is the evidence that allows the reconstructionof the events outlined here? The answer to boththese questions is held within the bedrock of theislands. The bedrock of Orkney is very much inevidence where the sea meets the land and therocky foundations are laid bare by the action ofthe sea.

When we look at the cliffedges of the islands, we seenear horizontal layers of rockpiled one upon the other.These rock layers onceextended from island toisland, during a time when thearchipelago and the mainlandof Caithness existed as acontinuous landmass. Theaction of sea and the ice overthe ages has worn away thislandmass, leaving behind theislands which are still beingeroded by the sea; thenumerous caves and seastacks are evidence of thiscontinuing erosion.

These generally flat-lying rock layers, which persistunderneath the land and dictate the low-lyinglandscape of Orkney, are popularly known as the‘Old Red Sandstone’. As well as sandstones thereare also layers of siltstone and mudstone. Owingto the ease with which the stone can be used forbuilding houses and walls, this naturalbuilding materialis also knownas ‘flagstone’.

Devonian 400 million years ago

Triassic 200 mya

Jurassic 135mya

Cretaceous 65mya

cross section of ancient sand dunes, Yesnaby

This Devonian landscape and environment wouldhave been totally unlike that of today, because400 million years ago Scotland and the rest ofBritain, was part of a continent called ‘Laurussia’that also included north America, Greenland andnorthwestern Scandinavia. This continent waslocated south of the equator. Stresses and strains

flagstone sea caves

WestMainlandAnticline

EdaySyncline

NorthScapaFault

East

Scap

aFau

lt

sea arch

Caithnessflagstone

Orkneysandstone

basement Orkneyrocks

faultline

Permian 225 mya

Orkney

equator

Please remember:To take care on the cliffs as they can be dangerousTo avoid disturbing nesting birdsTo keep dogs under control at all timesTo take your litter home with youTo not pick wild flowersTo respect private property

Westray

Papa WestrayNorth Ronaldsay

Sanday

EdayStronsay

Shapinsay

Rousay

Hoy

Flotta Burray

South Ronaldsay

Copinsay

Graemsay

Egilsay

Wyre

Gairsay

Swona

Orkney

Mainland

Stromness

Kirkwall

Auskerry

Scapa Flow

Yesnaby

1

23

45

Old Man of Hoy

Fossil Museum

Deerness

Orkney and thepossible extent of ice

and land mass 18,000 years ago

Orkney and thepossible extent of ice

and land mass 18,000 years ago

remains ofstromatolites

Other more unusual fossils known as stromatolitesalso occur and can be seen at Yesnaby 2 . These areseen as ocherous layers between some of theflagstones and form the ‘Horse tooth stone’ found ina sequence of rock layers known as the LowerStromness Flags. Stromatolites are the fossil remainsof blue-green algae that flourished in the beach areasof the ancient lake. The algae are considered to bepart of the army of agents that worked byphotosynthesis in very early geological time to producethe oxygen that supports us today. Also encased inthe rock, it is possible to see old worm burrows andworm casts of long ago, preserved through time inwhat used to be layers of soft sediment.

With the passage of time, the mountains around thelake became worn down, the lake and surroundinglowlying areas were filled in with sediment, andeventually, the whole area became a desert. Beneaththe ground surface the layer upon layer of sedimentthat had accumulated turned to rock. Millions ofyears later, tensions in the Earth crust, brought aboutby continuing continental drift, folded the rock layersand in places the rock sequence was sliced throughby geological breaks or faults.

There is also evidence ofvolcanoes, which you cansee at the south coast ofDeerness 3 , on the eastMainland. Here there is avolcanic plug, theremains of a volcano,together with its lava flowand layers of volcanicash. Volcanic lava andash also occur on thewest side of Hoy andunderlie the Old Man 4 .

fossil wave marks

fossilmud cracks

head of a fossil fish

fault showing rock sequence

plant fossilsThe fossil remains of plantsalso occur in the rock layersof Orkney. This indicatesthat despite the hot and aridclimate, there were plantsestablished on the marginsof the lake and associatedriver systems. These plantswere some of the earliestknown. When the plantsdied, fragments werewashed into the lake tobecome fossilised alongwith the fish.

The next 350 million years passed without furtherupheavals in Orkney until, around two million yearsago, global temperatures plummeted and the worldwas plunged into an ice age.

As recently as 17,000 years ago, the earth was still inthe grips of this ice age and, world-wide, sea levelwas more than 100 metres lower than it is today.Orkney was then a range of hills and valleys surroundedby low-lying land stretching as far as the Scottishmainland. Over the following 11,000 years,temperatures rose, ending the ice age and causingsea levels to rise. The land around Orkney was soonflooded, creatingan island.

Gradually the deeper valleys were also submerged,leaving only the hill tops and high ground standingclear of the water as islands. The sea level stabilisedabout 6000 years ago, by which time the islandslooked roughly as they do today. But like an unfinishedsculpture, they lacked detail. This was provided bythe force of the sea which even today continues tochisel away at the Orkney coastline. On inner, shelteredcoasts, the landscape is generally rather gentle, buton the exposed, westerly coasts of Hoy and Mainland,the rocks have borne the full brunt of the weather.Here, waves driven thousands of miles across theAtlantic have slowly scoured away the sandstone,producing the highest perpendicular cliffs in Britain,at St John’s Head on Hoy, which towers 346 metresabove the sea.

The variation in hardness of rock causes erosion tooccur at different rates. Headlands have formed wherethe rock is harder, bays where it is softer. Erosion hasalso shaped sea-stacks and geos. Geos are deepindentations in the cliffs formed where the waves havescoured and hollowed out fractures in the sandstone.

fossilwormcasts

The rock layers not only reveal the environmentalconditions of the ancient environment in which thefoundations of Orkney were formed, but also discloseother interesting information from the past. In manyof the islands, the fossil remains of the life that livedin and around Lake Orcadie can be found within therock layers. Most famous of the Orkney fossils arethe fish. Sometimes found complete, but morefrequently as fragments of scale and bone, the fossilfish suggest that Lake Orcadie teemed with life.

The changing environmental conditions echoed inthe rock layer sequences can be seen at variousplaces in Orkney. On Eday 1 , the change throughtime from layers of silty sediment that accumulatedin the middle of the lake, to sand and gravel riverdeposits, and then a return to lake deposits, is wellillustrated. At Yesnaby 2 , on Mainland, you can seethat in the cliffs, the rock layers were once sanddunes. Other features of the ancient environmentinclude ancient dried mud, in which you can clearlysee the sediment filled cracks, resulting in a criss-crossed pattern on the surface of some rock layers.

Some of the fish lived entirelyon the lake bottom, where theyscavenged for food. These fishtended to have boney armourplating to protect themselvesfrom fast-swimming predators.There were also other groupsof fish, precursors of thespecies we see on thefishmonger’s slab today, thatswam in shoals.

Following algal blooms and other naturalenvironmental factors that ‘poisoned’ the lake waters,fish mass mortalities seem to have occurred fromtime to time. The fish bodies drifted from near lakemargins, where they tended to live, towards thedeeper part of the lake where they sank to the bottomand were covered with fine silt and subsequentlyfossilised. Rock layers particularly rich in the remainsof fossil fish are known as ‘fish beds’. The fossilmuseum in Burray 5 has many fossils from Orkneyand elsewhere.

blowhole

stack

sea cave

gloup

geo

sea arch

headland

wavecutplatform

Orkney

Shetlandice cap

Scottishice cap

Scandinavianice cap

photo

grap

hs: S

NH

, Iain

Ash

man

desig

n an

d illu

stration: Ia

in A

shm

an

find out more about ourrich and varied islands

geos

weathered sandstone

cliff scenery

cover im

ages: to

p fo

ssil fish b

otto

m se

a a

rch

Orkney’snaturalheritageOrkney’snaturalheritage

Orkney’snaturalheritageOrkney’snaturalheritage

Orkney’sGeology Geology

Europe and ScotlandMaking it work together

Where there are now seventy or so islands inOrkney, dotted over an area of sea that liesbetween the North Sea and the Atlantic, thereused to be a great freshwater lake. This once vastlake existed nearly four hundred million yearsago, during the Devonian geological period, andstretched from what we now know as the MorayFirth, across Caithness to Orkney, Shetland andbeyond to the Norwegian coast. Given the name‘Lake Orcadie’ by geologists, this huge lake wasringed by high mountains, the ancestors of theGrampian and Northern Highlands. The mountainswere drained by mighty river systems thattransported enormous amounts of sediment intothe low lying areas. Sand and other loose sedimentthat did not reach as far as the lake, was blownaround by the wind to form sand dunes that wouldhave migrated across what was a hot, arid, desert-like landscape.

These layers represent the sediment that waswashed into Lake Orcadie from the surroundingmountains. It is thought that muds and sands toa depth of 4,000 metres were deposited and piledup in the lake and its surroundings. As timepassed and as environmental conditions changedover thousands and millions of years, the lakelevel fluctuated and areas which were previouslydry land were engulfed by water. At other times,the lake almost dried up completely.

in the crust of Devonian Scotland, induced bycontinental drift, sometimes gave rise to volcanicactivity, with the eruption of lava and ash overthe landscape from volcanoes. It was a result ofcontinental drift over the subsequent few hundredmillion years, that brought about Scotland’snorthward drift to its present latitude and thesplit between Britain and America with theformation of the Atlantic Ocean and North Sea.

Where was Orkney within this ancient environment?What is the evidence that allows the reconstructionof the events outlined here? The answer to boththese questions is held within the bedrock of theislands. The bedrock of Orkney is very much inevidence where the sea meets the land and therocky foundations are laid bare by the action ofthe sea.

When we look at the cliffedges of the islands, we seenear horizontal layers of rockpiled one upon the other.These rock layers onceextended from island toisland, during a time when thearchipelago and the mainlandof Caithness existed as acontinuous landmass. Theaction of sea and the ice overthe ages has worn away thislandmass, leaving behind theislands which are still beingeroded by the sea; thenumerous caves and seastacks are evidence of thiscontinuing erosion.

These generally flat-lying rock layers, which persistunderneath the land and dictate the low-lyinglandscape of Orkney, are popularly known as the‘Old Red Sandstone’. As well as sandstones thereare also layers of siltstone and mudstone. Owingto the ease with which the stone can be used forbuilding houses and walls, this naturalbuilding materialis also knownas ‘flagstone’.

Devonian 400 million years ago

Triassic 200 mya

Jurassic 135mya

Cretaceous 65mya

cross section of ancient sand dunes, Yesnaby

This Devonian landscape and environment wouldhave been totally unlike that of today, because400 million years ago Scotland and the rest ofBritain, was part of a continent called ‘Laurussia’that also included north America, Greenland andnorthwestern Scandinavia. This continent waslocated south of the equator. Stresses and strains

flagstonesea caves

WestMainlandAnticline

EdaySyncline

NorthScapa Fault

EastScap

aFau

lt

sea arch

Caithnessflagstone

Orkneysandstone

basement Orkneyrocks

faultline

Permian 225 mya

Orkney

equator

Please remember:To take care on the cliffs as they can be dangerousTo avoid disturbing nesting birdsTo keep dogs under control at all timesTo take your litter home with youTo not pick wild flowersTo respect private property

Westray

Papa WestrayNorth Ronaldsay

Sanday

EdayStronsay

Shapinsay

Rousay

Hoy

FlottaBurray

South Ronaldsay

Copinsay

Graemsay

Egilsay

Wyre

Gairsay

Swona

Orkney

Mainland

Stromness

Kirkwall

Auskerry

Scapa Flow

Yesnaby

1

23

45

Old Man of Hoy

Fossil Museum

Deerness