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Sedmihori / The Seven Mountains

A cache by 100Dteam Send Message to Owner Message this owner
Hidden : 6/7/2010
Difficulty:
2 out of 5
Terrain:
2 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

cz flag Sedmihori je vybornou ukazkou ringove intruze, jednou z nejlepsich v cele stredni Evrope. Ukol: zmerit prumer skalni misy (te horni).
gb flag Sedmihori is an excellent example of a ring intrusion, one of the best in the whole of Western Europe. Task: measure the diameter of the rock bowl (on the top).


pohled na Sedmihori
pohled na Sedmihori / view on the Sedmihori
czech flag Listing v Ceském jazyce

Sedmihori

Sedmihori je soustava kruhove rozmistenych kopcu, prevysujicich okolni teren o 100 – 150 metru. Cely masiv ma v prumeru 4 – 5 km a nachazi se v Zapadnich cechach, asi 10km severne od Horsovskeho Tyna. Sedmihori je vybornou ukazkou ringove intruze, jednou z nejlepsich v cele stredni Evrope. Jedna se take o uzemi s nejlepe vyvinutymi formami zvetravani zuly v cechach.

Prstencove struktury jsou obecne povazovany za smisene vulkanicko-plutonicke komplexy, prime dukazy o povrchovych projevech vulkanicke cinnosti u nich vsak casto chybeji.

Geologicke slozeni

Nákres Sedmihorí
nákres Sedmihorí / sketch Sedmihorí

Od okraje do stredu celeho granoidniho masivu se podstatne meni jeho mineralni slozeni.

  • Vnejsi cast je tvorena porfyrickou biotitickou zulou svetle sede barvy. zula je stredne zrnita s vysokym obsahem draselneho zivce (misty az 50%). V zakladni hmote je pritomen kremen, plagioklas (oligoklas), biotit a zridka muskovit.
  • Vnitrni cast tvori biotiticko-muskoviticka zula, belave az okrove sede barvy, stredne az hrube zrnita. Hlavnim rozdilem oproti vnejsi casti je stala pritomnost muskovitu a pouze ojedinela pritomnost draselneho zivce. zivce v teto zule byly slabe postizeny druhotnymi premenami (kaolinizace, sericitizace), a proto hornina snadneji zvetrava. To podporuje pokles vnitrni casti.
  • Jadro intruze tvori turmalinicko-muskoviticka zula. Je to drobnozrnna belave seda hornina s drobnymi castmi draselneho zivce s nizkym podilem slid.

Hranice mezi temito tremi castmi jsou pomerne ostre, ale presto maji prechodny raz. Svym slozenim se tyto zuly priblizuji cinonosnym zulam v Krusnych horach.

Vznik masivu

kameny

Zvetravani stredni casti by vsak samo o sobe nestacilo k vzniku takto vyrazne modelace terenu. Vyrazne vystupovani Sedmihori ma vysvetleni tektonicke. Zatimco cela oblast zapadne od marianskolazenskeho zlomu poklesla, Sedmihori zustalo v puvodni vysi nebo se mirne zvedlo. Poklesy okolniho krystalinika se odehraly podel rady mladsich zlomu, z nichz nejvyraznejsi je prokazan na jiznim okraji masivu.

Jednotlive kopce, tvorici Sedmihori, jsou oddeleny pomerne hlubokymi sedly. Jejich vznik je zpusoben vyskytem puklin, ktere prostupuji zulovy masiv. V mistech, kde jsou pukliny u sebe, dochazi k rychlejsimu rozpadu horniny a vzniku hlubokych sedel. Ze stejneho duvodu je cely prstenec otevren na vychodni strane. Tato strana byla jiz drive narusena marianskolazenskym zlomem a proto zde dochazi k rychlejsi erozi.

Stari sedmihorskeho pne je radiometricky datovane na 315-280 milionu let a jeho vznik nepochybne souvisi s variskym vrasnenim.

Sedmihori tedy mohlo byt vulkanickym centrem v obdobi svrchniho karbonu. O projevech vulkanismu z te doby mame radu dukazu v sedimentech plzenske, radnicke i mnoha dalsich jezernich panvi karbonskeho stari.

Projevy zvetravani, ktere zde naleznete

Skalní mísa
Skalní mísa / Rock bowl

Skalni misy

Kruhovite nebo ovalne prohlubne na hornich castech plochych skalnich masivu se nazyvaji skalni misy. Vyskytuji se na vsech typech hornin, nejcasteji vsak na piskovcich. Proto se s nimi take nejcasteji setkavame v piskovcovych skalnich mestech. Vznikaji v mistech mensi odolnosti horniny, pusobenim chemickeho i fyzikalniho zvetravani. Uplatnuje se zde cinnost lisejniku, mechu, mrazu i atmosferickych vlivu. Vetsina skalnich mis zadrzuje destovou vodu. Tato voda se postupem casu stava vuci hornine velmi agresivni, nebot vlivem slunecniho osvitu a obsahu mikroorganismu meni svoje pH. Vlivem vsech techto faktoru je objem skalni misy stale rozsirovan a vznikaji tak prohlubne az nekolik decimetru v prumeru.

Mrazove sruby

Mrazove sruby jsou skalni steny, ktere vznikly roztrhanim a rozpadem hornin podel prasklin. Podminkou vzniku Mrazovych srubu, jsou dve na sebe priblizne kolme praskliny. Skrz tyto praskliny prostupuje voda a soustreduje se zde. Zmrznutim zadrzene vody vznikaji velke tlaky, ktere nadzvedavaji material nad sebou. Pri rozmrznuti bloky poklesaji o neco nize nez se puvodne nachazely. Mnohonasobnym opakovanim tohoto deje dochazi k trhani a ustupu skalni steny. Takto utrzene bloky postupne sestupuji ze svahu a na jejich puvodnim miste zustava strma stena - mrazovy srub. Pod nim se vytvareji mirne uklonene kryoplanacni plosiny, pokryte suti z rozpadlych skalnich bloku, tzv. kamenna more.

Pseudokary

Pseudokary maji na rozdil od mrazovych srubu obloukovy prubeh. Vznikaji stejnymi procesy jako mrazovy srub, avsak skalni masiv musi mit pukliny ve vice smerech. Pro vznik pseudokaru jsou potreba minimalne ctyri steny. Pseudokary jsou nejen typicke pro toto uzemi, ale i velmi pekne na pohled.

Vase ukoly

Aby jste si mohli zalogovat tuto cache, musite splnit nasleduji ukoly. Nezapomente si vzit metr!

  • Vasim ukolem je zmerit prumer Skalni misy (te horni), kterou naleznete na vychozich souradnicich - (waypoint Sklani misy - N 49° 37.588 E 012° 52.565). Az na misto Vas take dovede modra turisticka znacka. Namerenou hodnotu odeslete na nas mail, uvedeny v nasem profilu.
  • Dobrovolny ukol - prilozte fotografii pseudokaru mrazoveho srubu, kamenne misy, kamenneho more nebo jakehokoliv zajimaveho utvaru, ktery pri Vasi navsteve Sedmihori naleznete.
  • Vyfotte se u nektereho z mrazovych srubu - (napr. waypoint Mrazovy srub - N 49° 37.268 E 012° 51.379). Viz ukazkove foto. Tato podminka byla zrusena aktualizaci pravidel dne 1.1.2011

Cache muzete logovat ihned, neni potreba cekat na potvrzeni spravnosti odpovedi



gb flag Listing in English

Sedmihori / The Seven Mountains

Sedmihori is a circular arrangement of hills, surmounting the surrounding terrain by 100-150 metres. The whole massif has a diameter of 4 - 5Km and is located in Western Bohemia, about 10Km north of Horsovsky Tyn. Sedmihori is an excellent example of a ring intrusion, one of the best in the whole of Western Europe. It is also an area with the most developed forms of weathered granite in the Czech Republic.

Ring structures are generally thought to be mixed volcanic-plutonic formations, however, direct proof of nearby surface volcanic activity is often missing.

Geological composition

Nákres Sedmihorí
nákres Sedmihorí / sketch Sedmihorí

The mineral composition of the whole granoid massif changes dramatically from its edges to its centre.

  • The outer part consists of light grey porphyric biotitic granite. The granite has a medium grain with a high content of potassium feldspar (in some parts up to 50%). Granite contains quartz, plagioclase (oligoclase), biotite and, rarely, muscovite.
  • The inner part consists of biotitic-muscovitic granite with a white to ochre-grey colour and medium to rough grain. The main difference between this and the outer part is the stable presence of muscovite and only a trace of potassium feldspar. Feldspars in this granite were slightly affected by secondary changes (kaolinization, sericitization) which are why the rock succumbs more easily to weathering. This supports the sinking of the inner part.
  • The core of the intrusion consists of tourmaline-muscovitic granite. This is a finely grained white-grey rock with minute parts of potassium feldspar and a low content of mica.

The borders between these three parts are relatively sharp, however, despite this there is a gradation between them. The content of these granites makes them very similar to tin-containing granites in the Krušný Mountains.

Massif formation

kameny

Weathering of the middle part would not have been sufficient, however, for the formation of such distinct terrain. The distinct shape of Sedmihorí is due to tectonics. While the whole area west of Mariansbad fault sank, Sedmihorí remained at the same elevation or was slightly raised. Sinkings of the nearby crystalinic areas occurred parallel to the younger fault lines, most notably on the southern edge of the massif.

The hills of Sedmihorí are divided by relatively deep saddles. They were formed due to the presence of cracks throughout the granite massif. The breakdown of rock in areas where cracks are close to each other is quicker than in other areas and results in the formation of deep saddles. For this reason there is an opening on the east side of the ring formation. This side was affected by the Mariansbad fault which is why it is subject to faster erosion.

The age of Sedmihorí is radiometrically dated to be 315-280 million years and its formation is undoubtedly linked to the Variscan orogeny.

Therefore, Sedmihorí could have been a volcanic centre in the late Carboniferous (Pennsylvanian) period. We have much evidence supporting volcanic activity from Pilsen, Radnice and many other lake basin sediments from this period.

Examples of weathering patterns which you will find here

Skalní mísa
Skalní mísa / Rock bowl

Rock bowls

Circular or oval depressions on the upper parts of flat rock massifs are called rock bowls. They are found on all types of rocks, however, they are most commonly found on sandstone. This is why we often find them on limestone rock formations. They form in areas where rock has been weakened due to chemical and physical weathering – caused by lichen and moss activity, freezing and atmospheric factors. Most rock bowls collect rainwater. This water becomes a serious factor in the weathering of rock bowls as sunlight and the presence of microorganisms change its pH. This causes the rock to widen over time which is how rock bowls as wide as several decimetres are formed.

Frost weathering

Frost weathering are rock walls which formed by the tearing and breaking down of rock along crack lines. Two approximately perpendicular cracks are needed for the formation of rock weathering. Water gets into the cracks and stays there. The freezing of this water creates great pressure as water expands when it freezes causing the rock either side of the crack to be pushed apart. After the water melts, the rock moves down a little further than it originally was. Repetition of this causes tearing and recession of the rock wall. Blocks of rock which are torn off this way gradually move downward and in their place is a steep wall – Frost weathering. Crayoplanation plateaus are formed under this, covered with ruins of broken blocks of rock, a so-called scree.

Pseudokar

In contrast to Frost weathering, Pseudokars have a curved shape. They are formed by the same processes as Frost weathering but the rock must have cracks in various different directions. A minimum of four walls are needed for the formation of Pseudokars. Pseudokars are not only typical for this country but also beautiful to look at.

Your tasks

In order for you to be able to log this cache, you must fulfil the following tasks. Don’t forget your tape measure!

  • Your task is to measure the diameter of the rock bowl (on the top) which you find at the starting point which has the following coordinates (waypoint rock bowl - N 49° 37.588 E 012° 52.565). A blue tourist sign will also lead you to this starting point. Send the rock bowl measurement to us via the email address on our profile.
  • Optional task – attach a photograph of a pseudokar, frost weathering, stone sea etc.
  • Take a photo of yourself at one of the Frost weathering – (e.g. waypoint Frost weathering - N 49° 37.268 E 012° 51.379). See sample photo. Edit because of new rules from 1.1.2011

You may log the cache immediately; there is no need to wait for confirmation that your answer is correct.

Za preklad velmi dekujeme nasi kamaradce Sente Shand.

Additional Hints (Decrypt)

armncbzra zrge

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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