Orø 55: Orøstenen Earth Cache EarthCache
Hidden : 7/9/2015
Difficulty:
1.5 out of 5
Terrain:
1.5 out of 5

Size: Size: other (other)

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Geocache Description:


Dette er en Earthcache og har ingen fysisk log. Se nedenfor hvordan du logger den.
Please observe, Earthcaches does not have a physical log.



Dette klippestykke kommer fra Bornholm og blev transporteret hertil af istidens gletsjere. Læg mærke til rillerne i toppen af stenen: Hvordan mon de er kommet?

Weichsel Istiden

Danmark var delvis dækket af is under seneste istid, Weichsel-istiden, der fandt sted mellem 110.000 og 15.000 år før nu. Under istiderne bevægede enorme gletsjere sig ned over Danmark. Man skal forestille sig et bjerg af is, hundrede meter højt nogle steder, der har "høvlet" ganske ganske langsomt hen over området her, ovre fra Bornholm, og har slæbt store sten, jord, sand og grus med som har hængt fast i isen.

Weichsel-istiden varede ca. 100.000 år og perioden var generelt kold og omfattede flere isfremstød fra nord til det danske område, men der var også lange perioder i Weichsel, hvor landet var isfrit. Da isens udbredelse var størst i Weichsel var omkring 31% af Jordens landareal dækket af gletsjere og havspejlet lå 100 meter lavere end nu.

De vigtigste isfremstød i Weichsel – se figuren:
1) Norske-isen, 2) Nord-Øst-isen, 3) Det Baltiske isfremstød.
(Kilde: www.geolex.dk)

På dette link findes animationer af isbevægelserne i Weichsel.
Under isens fremstød har nogle sten ligget mast fast i jorden og nogle sten været frosset fast i isen. Når gletsjeren har flyttet sig hen over jorden har disse sten undertiden skuret hen over hinanden og er blevet udsat for et enormt slid, som har efterladt dybe ridser. Mon det er denne process der er skyld i rillerne i Orø-stenen?

Da isen smeltede, efterlod den alt dette i lag, som kaldes for istidsdannelser. Disse lag udgør med få undtagelser overfladen i Danmark. (De væsentligste undtagelser er flint og kalk). De sten vi finder i istidslagene stammer for en meget stor del fra Skandinavien. Enkelte af disse sten har et så karakteristisk udseende, at man kan sige præcis, hvor de kommer fra. Det drejer sig om ca. 0,2% af stenene. Man kalder sådanne karakteristiske sten for ledeblokke. Orøstenen har ikke status af ledeblok, men man er næsten helt sikker på at den kommer fra ca. 1690 millioner år gammel gnejs fra Bornholm.

Gnejs

Gnejs er en grovkornet, stribet eller båndet grundfjelds-bjergart. Stribning og bånding skyldes deformation, hvilket viser, at gnejs er en metamorf bjergart, dvs. en bjergart, som oprindelig bestod af andre, såkaldte primære bjergarter. Den er dannet af kvartrige sedimentære eller vulkanske bjergarter, der er blevet udsat for ekstremt høje temperaturer og tryk. Gnejs dannes i vore dages kollisionsbjergkæder, som fx Himalaya, og er et vidnesbyrd om gamle bjergkædesystemer, der for længst er eroderet væk.

For Orøstenes vedkommende er det uvist om den er omdannet granit eller omdannet leret sandsten. (Kilde: Søren Floris, Geologisk Museum).

Bornholmsk gnejs tilhører geologisk Det Baltiske Skjold, hvis fortsættelse findes i SØ-Sverige, og som blev dannet for ca. 1,8-1,5 mia. år siden. Bornholmsk gnejs er stribet, grå og rødlig bjergart med partier af andre bjergarter: amfibolit, glimmerskifer og kvartsit. Hovedmineralerne er kvarts, alkalifeldspat og plagioklas samt biotit, hornblende og magnetit. Bornholmsk gnejs indeholder desuden blandingsbjergarter, migmatitter, der fremstår med flammer og årer af lyse mineraler i gnejsen.

Billedet viser et eksempel på Gnejs-bånd fra bornholmsk gnejs. På Orøsten kan man se bånd på toppen og syd-øst siden af stenen. Erosion er en proces, hvor vand og vind langsomt nedbryder klippen. Nogle gange ser man at erosion nedbryder nogle lag hurtige end andre. Mon det er erosion, der er skyld i rillerne i Orø-stenen?

Orøstenen

Orøstenen blev fundet ved Lindhøjgård i 1983 af ejeren. I 1996 blev stenen transporteret ved hjælp af en stor blokvogn til sin nuværende plads på højdedraget ved svinget på Næsbyvej. Herfra er der en af øens bedste udsigtspunkter over det skønne fjordlandskab, hvor man kan se vand i tre retninger.



Opgave:
Fra syd-øst siden hele vejen over toppen til nord siden løber nogle tydelige riller.
1. Beskriv rillerne med dine egne ord. Bredde? Dybde?

Lad os prøve at finde ud af hvordan rillerne er kommet:
2.På toppen og løber rillerne parallelt med stenens naturlige lag, hvilket kunne tyde på at rillerne kunne skyldes.... hvad?

3. Men se nu hvad rillerne gør på nordsiden. Hvad? Kan du nu sige hvad rillerne skyldes?

Gnejss-båndene på stenen:
4. Båndene er lettest at se på syd-øst siden af stenen som farvede striber. Hvilken farve?

5. Hvilken gletsjer i Weichsel-tiden bragte blokken hertil?

6. Tag gerne et billede af din GPS/cykel/... foran stenen og post det med din "Found it" log. Du må også godt være med på fotoet. Vigtigt! Skriv ikke svarene til 1-5 i loggen.

7. Send dine svar til min profil på geocaching.com før du logger online. Du skal ikke vente på mit svar, log bare med det samme. Jeg kontakter dig, hvis der er problemer med dit svar.



This rock came all the way from Bornholm and was transported by the ice age glaciers. Can you see the grooves at the top of the stone: I wonder why they are there?

The Weichselian glaciation

Denmark was partly covered by ice during the last ice age, the Weichsel glaciations, which occurred between 110,000 and 15,000 years ago. During the ice ages huge glaciers moved down over Denmark. One can imagine a mountain of ice, hundreds of meters high in some places that have "planed" quite slowly over the area here. It came from Bornholm, and has dragged rocks, soil, sand and gravel that had been stuck in the ice.

Weichsel glaciation lasted about 100,000 years and the period was generally cold and included several phases of glacial advances from the north to the Danish area, but there were also long periods in Weichsel, when the country was free of ice. At the time where the ice extent was at its greatest, about 31% of Earth's land area was covered by glaciers and sea level was 100 meters lower than now.

The main glaciers in Weichsel - see figure: 1) Norwegian-ice, 2) North-East-ice, 3) the Baltic ice. (Source: www.geolex.dk) Animation of ice movement here.
During the glaciation some rocks were pressed firmly in the ground and some rocks were frozen in the ice. When the glacier moved, these rocks sometimes shed over one another and were subjected to enormous wear which has left deep scratches. Maybe that is the process that is causing the grooves in Orø stone?

As the ice melted it left all this in layers, called glacial deposits. With few exceptions these layers constitute the surface of Denmark. (The main exceptions are flint and lime stone). The rocks we find in the ice age deposits mostly come from all over Scandinavia. These are called glacial erratics. Some of these rocks have such a distinctive look that it is possible to say exactly where they came from. This is true for approx. 0.2% of the glacial erratics. The Orø Rock does not have the status of guide block, but geologists are almost sure that it came from some 1690 million years old gneiss from Bornholm.

Gneiss

Gneiss is a common of rock formed by high-grade metamorphic processes, i.e., a rock, which originally consisted of other, so-called primary rocks. It is formed from quartz-rich sedimentary or volcanic rocks that have been exposed to extremely high temperatures and pressures. It is often foliated (composed of layers of sheet-like planar structures). The foliations are characterized by alternating darker and lighter colored bands, called “gneissic banding”. Striations and banding due to deformation, which shows that the gneiss is a metamorphic rock,. Gneiss formed in today's collision mountain ranges such as the Himalayas, and is a testimony to the ancient mountain range systems that have long since eroded away.

For the Orø rock it is not known whether it metamorphised from granite or clay sandstone. (Source: Soren Floris, Geological Museum).

Gneiss from Bornholm geologically belongs to the Baltic Shield, whose continuation is found in SE-Sweden, which was formed about 1.8 to 1.5 billion years ago. Bornholm gneiss is striped, gray and reddish rock with lots of other rock parts: amphibolites, mica slate and quartzite. The main minerals are quartz, alkalifeldspat and plagioclase and biotite, hornblende and magnetite. Bornholm gneiss also includes migmatitt that appear with flames and veins of bright minerals in the gneiss.

Gneiss is often characterized by alternating darker and lighter colored bands, called ”gneissic banding”. The picture shows an example of Gneiss bands on gneiss from Bornholm. On the Orø rock you can see bands on the top and south-east side of the rock. Erosion is a process where water and wind slowly breaks down the rock. Sometimes we see the erosion deplete some layers more rapid than others. Could it be erosion that caused the grooves in the Orø rock?

The Orø rock

The Orø rock was found near Lindhøjgård in 1983 by the owner. In 1996 the stone was transported by truck to its current place on the ridge at the turn on Næsbyvej. From here it is one of the island's best viewpoints of the beautiful fjord landscape, where you can see water in three directions.



Your task:
From the south-east side all the way over the top to the north side runs some distinct grooves.
1. Describe the grooves with your own words. Width? Depth?

Let us try to find out how the grooves have come:
2. At the top and runs grooves parallel to the natural layers, which might indicate that the grooves could be due to.... what?

3. Now look how the grooves runs on the north side. What happens here? Can you now tell what caused these grooves?

Take a look at the gnejss bands:
4. They are easiest to see on the south-east side of the rock as stripes. What color?

5. From which glaciers in Weichsel was the Orø rock transported by?

6. Optional: It would be great if you take a picture of your GPS/bike/dog ... in front of the stone and post it with your "Found it" log. You may also be in the photo. Important! Do not write answers to 1-5 in the log.

7. Send your answers to my profil at geocaching.com before logging online. You need not wait for my answer to log your find. I will contact you if there is a problem with your answers.

Additional Hints (Decrypt)

Xbbeqvangrear crtre vxxr ceæpvf cå fgrara ns urafla gvy qra genqvgvbaryyr pnpur. / Gur pbbeqvangrf qbrf abg cbvag cerpvfryl ng gur ebpx gb znxr ebbz sbe gur genqvgvbany pnpur.

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)


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