The rock lay­ers of Väs­ter­göt­lan­d’s tab­le mountains

The rocks in our tab­le mountains are sor­ted in lay­ers, kind of like the lay­ers of a cake. At the base is the so-cal­led base­ment rock, and at the top is a pro­tecti­ve cap of dole­ri­te. Between the­se igne­ous rocks are lay­ers of sedi­men­ta­ry rocks. The­se were for­med from sedi­ments depo­si­ted on the bot­tom of an anci­ent sea – sedi­ments that were sub­jec­ted to high pressu­re for mil­li­ons of years, tur­ning them into sto­ne. Thus, the tab­le mountains con­tain both fos­sils from long-extin­ct ani­mals and some of the world’s oldest mete­o­ri­te finds. All tab­le mountains do not have the same com­po­si­tion; they can dif­fer somewhat. Com­mon for all, however, is that they rest on the base­ment rock, so let’s start there.

Base­ment rock

The oldest bed­rock we have in Pla­tå­ber­gens Geo­park is 1,700 mil­li­on years old – such an immen­se­ly long time that we can hard­ly grasp it. This is what we call the base­ment rock, and it can be found bene­ath all of the tab­le mountains. The base­ment rock is com­po­sed of gra­ni­tes and gneis­ses that for­med as parts of an enor­mous mountain range. Around 600 mil­li­on years ago this mountain range had ero­ded down to a com­ple­tely flat sur­fa­ce, which we call the pene­plain. This flat sur­fa­ce can be found bene­ath both the tab­le mountains and the enti­re Väst­gö­ta Plain.

The pene­plain for­med across lar­ge parts of Swe­den but in most pla­ces it has been trans­for­med and bro­ken up. In the Geo­park, however, it is well pre­ser­ved and visib­le in many pla­ces, somet­hing that is uni­que in the world. For examp­le, the­re are lar­ge visib­le are­as at Nord­kro­ken and in Slätt­ber­gens natu­re reser­ve wit­hin the city of Trollhättan.

Sand­sto­ne

Around 550 mil­li­on years ago, our con­ti­nent was loca­ted south of the equa­tor. Here, on the bot­tom of a shal­low sea, sand was depo­si­ted onto the base­ment rock. This sand even­tu­al­ly beca­me the sand­sto­ne found in the tab­le mountains. We can still see tra­ces of the anci­ent san­dy sea­flo­or, as the­re are ripp­le marks cre­a­ted by the waves of the anci­ent sea as well as tracks from the aqua­tic orga­nisms that craw­led across the sand. Such ripp­le marks are visib­le at Trol­men har­bour by Kin­ne­kul­le and in the ceiling of the Mill­sto­ne Mine at Lug­nås­ber­get. Sand­sto­ne is a porous rock that some­ti­mes cre­a­te spectacu­lar for­ma­tions, espe­ci­al­ly along the sho­res at Kinnekulle.

Sand­sto­ne beca­me a com­mon buil­ding mate­ri­al in the 12th cen­tu­ry. Many of the chur­ches around Kin­ne­kul­le are built of sand­sto­ne, and so is the Ska­ra Cathedral.

Alum sha­le

Clay par­ticles depo­si­ted on top of the sand, toget­her with remains of dead orga­nisms, beca­me the rock we call alum sha­le. Alum sha­le has been an impor­tant resour­ce for humans – for good and bad. The alum sha­le con­tains oil and seve­ral impor­tant (and hazar­dous) mine­rals and metals.

Oil was extrac­ted at Kin­ne­kul­le during both World War I and II, when oil pri­ces were high. At Ran­stad on Bil­ling­en, ura­ni­um was extrac­ted in the 1950s and ‘60s. The alum sha­le also con­tains ‘balls’ of limesto­ne, cal­led stink­sto­ne, that were mined and bur­ned to extract lime. The rea­son for their name is that they emit a bad smell, remi­ni­scent of cru­de oil, when bro­ken open. The alum sha­le is a porous rock that easily crumbles.

Limesto­ne

On top of the clay, lime mud and shell remains from vari­ous aqua­tic orga­nisms (such as corals and gastro­pods) were depo­si­ted, for­ming the rock known as limesto­ne. The com­po­si­tion of the limesto­ne reflects the envi­ron­ment at the time of depo­si­tion; thus, it con­tains lar­ge amounts of fos­sils – for examp­le of ort­ho­ce­ra­ti­tes, pre­histo­ric cep­ha­lo­pods with long and nar­row, coni­cal shells. The limesto­ne at Kin­ne­kul­le also con­tains remains of the world’s oldest mete­o­ri­tes – rocks from spa­ce that struck the bot­tom of the shal­low sea.

Sin­ce Medi­ae­val times, limesto­ne has been used as buil­ding mate­ri­al in both chur­ches and cast­les. Later on, limesto­ne beca­me an impor­tant resour­ce as fer­ti­li­ser for farm­land. In the 19th cen­tu­ry, lar­ge limesto­ne quar­ri­es were opened as the rock began to be used as a com­po­nent in cement. The­re are plen­ty of tra­ces from the limesto­ne indu­stry all around the tab­le mountains. 

Ben­to­ni­te

The rock known as ben­to­ni­te can be found as a thin lay­er in our tab­le mountains, often only a couple of deci­metres thick – or a litt­le thic­ker in some are­as. Ben­to­ni­te is com­po­sed of vol­ca­nic ash that spre­ad across Earth during a dra­ma­tic peri­od in our planet’s histo­ry. The ash fell into the sea and sunk to the bot­tom as sedi­ment, even­tu­al­ly for­ming the clay mine­ral cal­led bentonite.

A spe­ci­al pro­per­ty of ben­to­ni­te is that it swells as it absorbs water. This pro­per­ty makes it use­ful as a sea­lant – for examp­le, it has been sug­ges­ted as a sea­lant around final dis­po­sal canis­ters for spent nuclear fuel. Sin­ce ben­to­ni­te can absorb flu­ids it is also used as cat litter.

Pho­to: Sven-Åke Larson

Clay sha­le

The clay sha­le also for­med from clay on the sea­flo­or, tur­ning to sto­ne during mil­li­ons of years of high pressu­re. The clay sha­le lay­er sits near the top of the tab­le mountains, direct­ly bene­ath the dole­ri­te cap. Clay sha­le often con­tains fos­sils, for examples of tri­lo­bi­tes and vari­ous plants. Anot­her type of fos­sil found in the clay sha­le is grap­to­li­tes, small colo­ni­al ani­mals that once lived in the clay on the seafloor.

Dole­ri­te

At the top of the tab­le mountains we find the igne­ous rock known as dole­ri­te – the very rea­son we have tab­le mountains in our regi­on today. It for­med from mag­ma rising from Earth’s mant­le around 280 mil­li­on years ago, which soli­di­fi­ed here and the­re insi­de the sedi­men­ta­ry rock lay­ers. Dole­ri­te is a hard rock that doesn’t ero­de easily. Whe­re the­re was no dole­ri­te, the sedi­men­ta­ry rocks ero­ded all the way down to the base­ment rock. But in the pla­ces whe­re the dole­ri­te had penetra­ted the sedi­men­ta­ry rocks, it for­med a pro­tecti­ve cap over the sof­ter rocks, and thus the tab­le mountains were even­tu­al­ly created.

On seve­ral of our tab­le mountains the dole­ri­te is espe­ci­al­ly visib­le whe­re it emer­ges with its ver­ti­cal cliff faces and spectacu­lar for­ma­tions. An old Swe­dish word for dole­ri­te is trapp, deri­ved from the word for ‘stairs’, as the dole­ri­te for­ma­tions are often remi­ni­scent of stairs.

Do you want to know more?

Con­ti­nents have col­li­ded, drif­ted apart and col­li­ded again. Vol­ca­no­es have rava­ged the regi­on. Weat­her, wind, and changes in tem­pe­ra­tu­re have ero­ded mountains. Kilo­met­re-thick ices have re-sha­ped the lands­cape. Immen­se floods have washed across the lands.

Why does the tab­le mountain lands­cape look the way it does? When and how did the base­ment rock form? What sea col­lec­ted the sedi­ments that beca­me mountains? When did the tab­le mountains form? How have ice ages affec­ted the landscape?

Find out more