The geo­lo­gy

Geo­lo­gy is histo­ry. The histo­ry of the earth. In the tab­le mountain lands­cape, we find geo­lo­gi­cal sites and phe­no­me­na that only exist here, or in a few pla­ces in the world. Here you can tra­vel in time — seve­ral mil­li­on years. Each pla­ce is a pie­ce of the puzz­le, which toget­her tell a sto­ry that begins 1700 mil­li­on years ago.

All around the glo­be, the con­ti­nents of the earth are in con­stant move­ment. Our planet’s sur­fa­ce is made up of moving tecto­nic pla­tes which can col­li­de or sli­de under­ne­ath each other, resul­ting in vol­ca­nic erup­tions. This is how oce­ans are born and die, how mountain ranges are built up and bre­ak down. It all takes very long time, of cour­se – mil­li­ons of years.

In West Swe­den, tecto­nic pla­tes have col­li­ded and joi­ned with an older pla­te to the east seve­ral times. The vari­ous rocks found in Pla­tå­ber­gens Geo­park have for­med deep wit­hin the earth but also at shal­low dept­hs, in con­nec­tion to vol­ca­nic erup­tions. The­re are also rocks here for­med from sedi­ment, which have become pressed toget­her under high pressu­re and high tem­pe­ra­tu­res, trans­for­ming them into stone.

Earth’s con­ti­nents are in con­stant move­ment around the glo­be. Our planet’s outer lay­er con­si­sts of pla­tes that move and can col­li­de or sli­de bene­ath each other, resul­ting in eart­hqua­kes and vol­ca­nic erup­tions. This is how oce­ans are born and die, how mountain ranges are cre­a­ted and worn down. Of cour­se, this all hap­pens across very long peri­ods of time – mil­li­ons of years. West Swe­den has seen seve­ral pla­te col­li­sions whe­re pla­tes have mer­ged with an older con­ti­nen­tal crust to the east. Some of the rocks in Pla­tå­ber­gens Geo­park have for­med deep down insi­de the earth, and some have for­med clo­ser to the earth’s crust. But we can also find rocks for­med from sedi­ment, which beca­me expo­sed to high pressu­re and tem­pe­ra­tu­res and thus tur­ned to stone.

Pla­tå­ber­gens Geo­park con­tains geo­lo­gi­cal phe­no­me­na and sites that are uni­que to this area or can only be found in few pla­ces around the world. Its geo­lo­gi­cal histo­ry stret­ches 1,700 mil­li­on years into the past! The following phe­no­me­na are of spe­ci­al inter­na­tio­nal interest:

  • The pene­plain
  • The tab­le mountains
  • Qua­ter­na­ry geo­lo­gy and ice age deposits.

 The Swe­dish name Pla­tå­ber­gen lite­ral­ly means “the Pla­teau Mountains”.

Down­lo­ad broschu­re – The oldest sto­ry ever told (PDF)
Down­lo­ad in-depth study – Geo­lo­gi­cal heri­tage (PDF)
Milesto­nes in the geo­lo­gi­cal histo­ry of the tab­le mountain landscape
  • 20,000–11,700 years: the ice she­et begins to withdraw. The Bal­tic Ice Lake is drai­ned in two cataclys­mic events.
  • 115,000 years: the latest ice age begins and res­ha­pes the lands­cape yet again.
  • 250 mil­li­on years and onwards: the sedi­men­ta­ry rocks ero­de, lea­ving only tho­se are­as cove­red by the dolerite.
  • 280 mil­li­on years: mag­ma from insi­de the earth rises and penetra­tes the sedi­men­ta­ry rocks in some pla­ces, cre­a­ting the rock known as dolerite.
  • 540 mil­li­on years: the sea covers the pene­plain. Sedi­ment is depo­si­ted on top of the pene­plain, even­tu­al­ly becoming the sedi­men­ta­ry rocks in the tab­le mountains.
  • 600 mil­li­on years: the bed­rock ero­des and even­tu­al­ly becomes a peneplain.
  • 1,700 mil­li­on years: for­ma­tion of the base­ment rock.

The oldest bed­rock, the mylo­ni­te zone, and the peneplain

Did you know that the oldest rock types found in Pla­tå­ber­gens Geo­park for­med 1.7 bil­li­on years ago? That is such an incre­dib­ly long per­specti­ve com­pa­red to human histo­ry that we strugg­le to understand it … The oldest rocks for­med befo­re the­re were any advan­ced life forms on Earth – no soil, no tre­es, no ani­mals. The rocks were thus extre­mely expo­sed to a lot of stress from the weat­her, and over 600 mil­li­on years they were ero­ded down to a com­ple­tely flat sur­fa­ce – the pene­plain. Whe­ne­ver you walk on the pene­plain, you are trans­por­ted back in time to Earth as it was befo­re life. 

The base­ment rock

The area around Pla­tå­ber­gen in the pro­vin­ce of Väs­ter­göt­land sits on top of the base­ment rock, a part of the earth’s crust that for­med approx­i­ma­tely 1.7–1.65 bil­li­on years ago. At this time, two tecto­nic pla­tes met and hot, mel­ted rock (mag­ma) emer­ged from wit­hin the earth, for­ming the first bed­rock in Pla­tå­ber­gens Geo­park. It was made up of gra­ni­te, but the long and vio­lent histo­ry that this gra­ni­te has endu­red has cau­sed it to part­ly be trans­for­med into vari­ous forms of gneiss. Becau­se of their distin­ct appea­ran­ce, we say that the gneis­ses are foli­a­ted and banded.

Some types of gra­ni­te in the geo­park are dif­fe­rent from the 1.7‑billion-year-old foli­a­ted and ban­ded gneis­ses that make up the oldest bed­rock. The­se young­er rocks for­med approx­i­ma­tely 1.3 bil­li­on years ago when mag­ma penetra­ted the older bed­rock and cong­ea­led deep wit­hin it. The­se rocks have not had to endu­re the same events which defor­med and trans­for­med the older bed­rock into the ban­ded gneis­ses we can see today. Thus, the­se young­er gra­ni­tes look less defor­med than the gneis­ses. The­re is some ban­ding in the young­er gra­ni­tes, however, but that is the result of a col­li­sion with the North Ame­ri­can tecto­nic pla­te approx­i­ma­tely 1 bil­li­on years ago.    

The mylo­ni­te zone

Due to the move­ments in the earth’s crust, con­ti­nents have col­li­ded and sepa­ra­ted seve­ral times in the area that is known today as Väs­ter­göt­land. The pro­ces­ses which affect the lands­cape and the rocks in the area are mas­si­ve in sca­le and immen­se­ly power­ful. One event had an espe­ci­al­ly strong impact on the geopark’s oldest bed­rock: the col­li­sion with the North Ame­ri­can tecto­nic pla­te, which took pla­ce approx­i­ma­tely 1 bil­li­on years ago. Lar­ge chunks of the bed­rock were pus­hed in an eas­ter­ly direc­tion, almost doubling the thick­ness of the earth’s crust to the east. Becau­se of the weight of the pro­jecting bed­rock, the under­ly­ing part of the crust was pus­hed down­wards deep into the earth, whe­re the rock was hea­ted up and trans­for­med. One area whe­re this took pla­ce was along the so-cal­led mylo­ni­te zone, an area which can be follo­wed through the geo­park in a north-south direction.

When the bed­rock was pus­hed toget­her, it resul­ted in the for­ma­tion of a tall mountain range. As the pla­tes begun to move apart again approx­i­ma­tely 970 mil­li­on years ago, the mountain range col­lap­sed. The pro­jecting bed­rock then slid back west­wards (along the mylo­ni­te zone, for examp­le) and the crust, which had been pressed down, rose back up, weat­he­red and even­tu­al­ly regai­ned its nor­mal thick­ness. Thus, the sur­fa­ce we walk on in the geo­park today was once loca­ted at a depth of seve­ral kilo­metres. The sedi­ment which for­med from the weat­he­red mountain range, as it rose up after being rid of the weight of the pro­jecting bed­rock, can today only be found scat­te­red far out­si­de the area of the geo­park.        

The pene­plain

A litt­le over 600 mil­li­on years ago, a glo­bal coo­ling of the earth’s crust resul­ted in major gla­ci­a­tions. This event was follo­wed by a cli­ma­te which pro­mo­ted hea­vy ero­sion of the bed­rock. The lands­cape ero­ded to an even sur­fa­ce and lar­ge, flat are­as for­med, which are cal­led pene­plains. The pene­plain in the geo­park is known as the “sub-Cam­bri­an pene­plain”, sin­ce it for­med befo­re the Cam­bri­an Peri­od and is thus situ­a­ted bene­ath the Cam­bri­an sedi­men­ta­ry rocks in Platåbergen.

The pene­plain for­med over lar­ge parts of Swe­den and this anci­ent base­ment rock sur­fa­ce can be seen in seve­ral pla­ces around the geo­park. What we today know as Väst­gö­tas­lät­ten (the Väs­ter­göt­land plain) is in fact part of this weat­he­red base­ment rock surface. 

The mylo­ni­te zone (MZ) can be tra­ced from the nort­hern parts of the pro­vin­ce of Hal­land to Lake Mjö­sa in Norway.

Pene­pla­net at Nord­kro­ken — One of Swe­den’s oldest landscapes

The pene­plain at Nord­kro­ken seen from abo­ve at the wes­tern edge of the mountain on Halleberg

The tab­le mountains

Around 500 mil­li­on years ago, our con­ti­nent was situ­a­ted south of the equa­tor. In this envi­ron­ment, sedi­ment for­med on the bot­tom of a sea – sand, lime and clay. The­se sedi­ments later beca­me sto­ne buil­ding the tab­le mountains. The soft, sedi­men­ta­ry rocks have been pre­ser­ved thanks to a pro­tecti­ve cover made up of the much har­der rock known as dolerite.

The lay­ers of rock seen in Pla­tå­ber­gen are a win­dow into the geo­lo­gi­cal peri­od known as the Palae­o­zo­ic (541 to 251 mil­li­on years ago). The rocks here bear silent wit­ness to seve­ral geo­lo­gi­cal pro­ces­ses which are impor­tant for understan­ding our planet’s deve­lop­ment. For examp­le, both fos­sils of extin­ct spe­ci­es and some of the oldest mete­o­ri­te finds in the world can be found here, in the tab­le mountains of Västergötland. 

The super­con­ti­nent which had for­med 1 bil­li­on years ago, con­tai­ning more than Nort­hern Euro­pe and North Ame­ri­ca, split apart again approx­i­ma­tely 600 mil­li­on years ago. An oce­an, known as the Iape­tus Oce­an, for­med between the North Ame­ri­can con­ti­nent and Nort­hern Euro­pe. Sand par­ticles and cal­ci­um-rich sedi­ment were depo­si­ted in the oce­an, which itself was rich in aqua­tic orga­nisms – somet­hing that we can see today in the form of fos­sils in the dif­fe­rent lay­ers of Platåbergen.

As the gap between North Ame­ri­ca and Nort­hern Euro­pe widened, the oce­an expan­ded. Clay par­ticles were depo­si­ted on top of the sand along with remains of dead orga­nisms. The sand par­ticles later beca­me sand­sto­ne, and the clay depo­si­ted on the oce­an flo­or was the ori­gin of the rock type that we know as alum shale.

During the Ordo­vici­an Peri­od, the sea­flo­or spre­a­ding cea­sed and North Ame­ri­ca and Nort­hern Euro­pe once again began to move clo­ser toget­her. Lar­ge quan­ti­ti­es of cal­ci­um-rich sedi­ment were depo­si­ted in the Iape­tus Oce­an, sedi­ment which would even­tu­al­ly become limesto­ne. This type of limesto­ne has been mined in Häl­le­kis in Väs­ter­göt­land and is known in Swe­den as Orto­cer limestone.

When the North Ame­ri­can pla­te moved clo­ser to the equa­tor, the Iape­tus Oce­an con­ti­nu­ed to shrink and at the end of the Silu­ri­an Peri­od (approx­i­ma­tely 400 mil­li­on years ago), North Ame­ri­ca col­li­ded with the Nort­hern Euro­pe­an pla­te. This event cau­sed the for­ma­tion of the fells, or the Scan­di­na­vi­an Cale­doni­des. Resi­du­al sedi­ment from this shrin­king oce­an has been pre­ser­ved as a top lay­er of dark sha­le, for examp­le at Kinnekulle.

At the end of the Car­bo­ni­fe­rous Peri­od (approx­i­ma­tely 358 to 298 mil­li­on years ago), the earth’s crust rup­tu­red in the area whe­re the Nor­wegi­an capi­tal Oslo is now situ­a­ted, which resul­ted in sig­ni­fi­cant vol­ca­nic acti­vi­ty. This vol­ca­nic acti­vi­ty las­ted for tens of mil­li­ons of years and could also be felt in what is now West Swe­den. Insi­de the thick lay­ers of sedi­ment which cove­red the geo­park area, dark mag­ma emer­ged, rich in iron and mag­ne­sium. When the mag­ma reached the upper part of the crust, it see­ped in along the hori­zon­tal sedi­ment lay­ers and cong­ea­led, for­ming “pas­sa­ges” of dole­ri­te. The rela­ti­vely soft sedi­men­ta­ry rocks cove­ring the dole­ri­te ero­ded quick­ly and can today be found elsewhe­re, for examp­le in the Bal­tic Sea. Thus, the hard dole­ri­te was expo­sed and today we can see it at the top of the tab­le mountains.

Here’s a jing­le to remem­ber the rock lay­ers in the mountains. “USA KLoc­kan Tre” (“USA three o’clock”) stands for the Swe­dish names of the rocks in the correct order: bed­rock, sand­sto­ne, alum sha­le, limesto­ne, clay sha­le and dolerite.

When the mag­ma reached the upper part of the crust, it see­ped in along the hori­zon­tal sedi­ment lay­ers and cong­ea­led, for­ming “pas­sa­ges” of dolerite.

Qua­ter­na­ry geology

The tab­le mountains in Väs­ter­göt­land have part­ly been ero­ded down to their cur­rent shape due to seve­ral gla­ci­a­tions which have occur­red sin­ce the mountains for­med. The pre­sent-day lands­cape is lar­gely cha­rac­te­ri­sed by depo­sits and for­ma­tions cre­a­ted during the last ice age, which began 115,000 years ago. When the ice was at its thic­kest, the geo­park area was cove­red by an ice she­et approx­i­ma­tely 3.5 kilo­metres thick. As the cli­ma­te beca­me war­mer approx­i­ma­tely 20,000 years ago, the ice began to retre­at nort­h­wards (having reached as far south as the area around pre­sent-day Ber­lin in Ger­ma­ny) and Swe­den gradu­al­ly beca­me ice free.

During the mel­ting of the ice, seve­ral dra­ma­tic events took pla­ce in the geo­park area. A huge mel­t­wa­ter lake – the Bal­tic Ice Lake – was drai­ned twice into the Wes­tern Sea through a pas­sage direct­ly north of Bil­ling­en. Furt­her­mo­re, between the­se drai­nage events, an abrupt coo­ling of the cli­ma­te lasting more than a thousand years cau­sed the ice to grow back again.

The first drai­nage occur­red when the ice retre­a­ted, lea­ving a pas­sage direct­ly north of Bil­ling­en through which the Bal­tic Ice Lake, a dam­med-up lake situ­a­ted roughly whe­re the Bal­tic Sea is today, was drai­ned in a cataclys­mic event.

The cli­ma­te abrupt­ly coo­led down short­ly after the first drai­nage and in a mat­ter of years tem­pe­ra­tu­res were the same as during the ice age. This peri­od is known as the Young­er Dry­as (approx­i­ma­tely 12,800–11,600 years ago). The cold cli­ma­te cau­sed the ice to grow back, and the ice edge again moved south in the area of the geo­park. Lar­ge quan­ti­ti­es of gla­ci­al till, in the form of boul­ders and rocks, were depo­si­ted in front of the ice edge, cre­a­ting what we know as Hin­dens rev and Hjor­tens udde (“Reef of the Hind” and “Stag Point”). The­se are part of the so-cal­led Mid-Swe­dish Edge Zone, along with Råda Ås, Ska­ra­ryg­gen, and seve­ral other ridges. 

The next major event was the second sud­den drai­nage of the Bal­tic Ice Lake, which took pla­ce when the ice that had for­med during the Young­er Dry­as began to melt. The ice edge again retre­a­ted to the north end of Bil­ling­en. Then, in a sud­den cataclysm approx­i­ma­tely 11,700 years ago, the mel­t­wa­ter from the ice lake bro­ke trough the gla­ci­al edge and flooded the plains below. The Bal­tic Ice Lake, which con­tai­ned huge quan­ti­ti­es of mel­t­wa­ter, had grown as the ice edge retre­a­ted nort­h­wards. Sin­ce the ice lake was situ­a­ted 25 metres abo­ve sea level, the water was drai­ned west­wards into the North Atlan­tic. Cal­cu­la­tions show that 7,500 km3 of water rushed through the lands­cape! It took almost a year for all the water to drain from the Bal­tic Ice Lake to sea level, but even­tu­al­ly a con­ti­nu­ous sea for­med – the Yol­dia Sea.

The geo­lo­gi­cal histo­ry of the geo­park can also be seen in the com­po­si­tion of the vari­ous soil types. Are­as in the north and nort­hwest, which were once below the sea, are domi­na­ted by clay and other fine-grai­ned soil types, whi­le are­as in the sout­he­ast are made up of morai­nes, whe­re ero­ded sha­le from the tab­le mountains con­tri­bu­te to make the soil fer­ti­le. In some pla­ces with a limesto­ne bed­rock the soil cover is very thin. This type of land is the same as the alvar in Öland and it makes for extre­me far­ming conditions.