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Opuka

A cache by kulhal Send Message to Owner Message this owner
Hidden : 06/20/2010
Difficulty:
1.5 out of 5
Terrain:
2 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

CZ: Vychazka k odlehlemu opukovemu lomu
EN: A walk to a secluded "opuka" quarry. The English translation is below.

At uz prijedete ke Dzbanu z kterehokoliv smeru, s opukou se zcela jiste setkate. Tato specificka hornina totiz formovala okolni krajinu hned dvakrat. Za prve stoji za vznikem hlubokych udoli a  plochych nahornich planin jako je Rovina nad Kounovem, anebo planina, na kterou vas privede tato kes. Za druhe se opuka v uplynulych staletich stala oblibenym stavebnim materialem a domy postavene z neomitnutych belavych kvadru jsou symbolem zdejsiho kraje. Je zajimave, ze opuka je velmi mistni zalezitosti. Jen obtizne by se vam podarilo najit ve svete horninu se stejnymi vlastnostmi. Svedci o tom i to, ze pro opuku neexistuje jednoznacny termin v jinych jazycich. V nemcine je tento typ horniny nazyvan Flammenmergel, Schwammflitstein nebo Amberger Tripple, v anglictine pak spongilite, calcitic spongilite nebo cretaceous spongilite, nespravne pak i  marl, marlstone nebo clay marl. Jak komplikovane proti napr. vapenci (Kalkstein, limestone) nebo piskovci (Sandstein, sandstone).

Opuka krajinotvorna

Opuka patri mezi horniny usazene. Mezi hlavni slozky patri spongility (spongiliticke jilovce), ruzne formy kremicitych usazenin, zpevneny jil a ruzne mnozstvi kalcitu plniciho funkci pojiciho materialu. Opukove usazeniny vznikaly v druhohorach zhruba pred 90 miliony let na dne kridoveho more. Dokladem puvodu opuk jsou zkameneliny morske fauny a tvori je predevsim lastury a ulity mekkysu. Typickou primesi opuky jsou mikroskopicke jehlice morskych hub tvorene opalem (tzv. spongie - odtud obcas pouzivany nazev spongilit). Opukova vrstva je silna 3 - 12 metru. Sama o sobe je pomerne tvrda, lezi ale na mekkem permskem piskovci. Tento piskovec snadno zvetrava a po jeho odplaveni dochazi i k odlamovani svrchni opukove vrstvy. To vysvetluje podobu zdejsiho kraje: Celistva opukova vrstva nahornich planin je z okraju "ohlodavana" eroznimi vlivy. Jakmile jednou vznikne v tabuli zarez, eroze nabira na rychlosti, udoli se prohlubuji a rozsiruji. Jejich sklon zustava prudky, protoze jejich horni cast je opevnena opukou a nepoddava se erozi tak rychle jako mekci hornina pod ni. 

Lomova stena

Opuka mestotvorna

V Cechach je nekolik oblasti, kde se opuka tezila - predevsim v okoli Prahy (Bila hora, Petrin, nyni jediny cinny lom v Predni Kopanine), dale Dzbansko na Rakovnicku. V severnich Cechach jsou zname lokality v okoli Loun (Zemechy) a Zernosek. Nekolik mene vyznamnych lokalit se nachazelo i ve vychodnich Cechach. Opuka pro svoji lehkost a snadnou opracovatelnost byla v minulosti zakladnim stavebnim materialem vsech okolnich stredovekych mest, vcetne Prahy. Jsou z ni postaveny vsechny romanske rotundy, bazilika sv. Jiri na Prazskem hrade, klaster sv. Anezky ceske a mnohe dalsi vyznamne stavby. Opuka byla pouzivana rovnez jako socharsky material - pripomenme znamou keltskou hlavu z Mseckych Zehrovic nebo sarkofagy Premyslovcu v chramu sv. Vita. Krasny a historicky cenny je napriklad detsky opukovy nahrobek Premyslovny Guty II. (+ 1207), dcery Vaclava II., nalezeny v klastere sv. Anezky ceske na Starem meste. Z opuky je rovnez postavena rada vesnickych domu a kostelu v mistech jejiho vyskytu, tedy i ve vesnicich kolem Dzbanu. Mnohe z nich jste jiste zahledli cestou sem.


Rotunda v Predni Kopanine
Kostel sv. Mari Magdaleny, Predmi Kopanina
foto Martin Vavrik, Wikimedia Commons
Rotunda sv. Martina na Vysehrade
Rotunda sv. Martina, Vysehrad
foto Adam Zivner, Wikimedia Commons

O kesi

Nejpohodlnejsi pristup je od silnice spojujici Hredle a Treboc. Tam muzete zanechat sve auto (viz parkovaci waypoint). Od silnice jdete za sipkou po zlute znacene turisticke trase. Vsimnete si, ze jdete tesne po hrane nahorni plosiny. Na mnoha mistech staci jen par kroku vlevo do lesa a stojite nad eroznim srazem. Obcas se vam podari zahlednout udoli pod vami. Od jara do podzimu vsak vyhled byva zakryty korunami stromu, proto doporucuji navstivit vyhlidkove misto uvedene ve waypointech. Je to jen kousek od cilovych souradnic. Ty vas privedou na okraj jednoho z poslednich fungujicich opukovych lomu v Cechach. Vstup do lomu je zakazan navzdory tomu, ze prekazky vstupu jsou spise symbolicke. Lomovou stenu si muzete dobre prohlednout i z cesty.


Podminky pro zalogovani nalezu:
  1. Na emailovou adresu uvedenou v profilu mi prosim poslete odpovedi na nasledujici otazky:
    a) Jak opuka vznikla?
    b) Od ceho je podle vaseho nazoru odvozen nazev "opuka"?
    c) Shybnete se a vezmete do ruky jeden z mnoha opukovych ulomku. Dobre si jej prohlednete a ohmatejte. Popiste vysledky vaseho zkoumani.
    d) Jaka je podle vas pricina toho, ze z opuky postavene domy zustavaji casto neomitnute?
  2. Nepovinne: Ke svemu logu pripojte prosim vasi fotografii na vyse uvedenych souradnicich s lomem v pozadi.

Zdroje:
Kotlík, P.: Stavební materiály historických objektu, VSCHT, Praha, 1999
Wikipedia
Ceska geologicka sluzba
Rabasova galerie, Rakovník

Whatever direction you come from, you will surely see "opuka". This unusual rock has formed the country around Dzban twice: First, its presence led to the formation of deep valleys and flat table mountains - like the one where this earthcache is located. Second, opuka was the most popular construction material in past, and buildings from white stones are very typical for the villages in the surrounding area. Opuka is a very local, specific rock. It would be very difficult to find an identical mineral elsewhere in the world. You can see it even on its name in other languages: In German it is called  Flammenmergel, Schwammflitstein or Amberger Tripple. In English similar rocks are called spongilite, calcitic spongilite or cretaceous spongilite, incorrectly also  marl, marlstone or clay marl. How complex compare to e.g. limestone (Kalkstein) or sandstone (Sandstein).

Opuka shaping the country: Opuka belongs to sediments. Its main components are spongilits, various forms of silicate sediments, hardened clay and variable amounts of calcite that plays the role of a glue. Opuka sediments were formed in mesozoic era about 90 million years ago, on the bottom of cetraceous sea. The proof of opukas origin are fossils of sea fauna, especially molluscoids' shells. A typical component are also microscopic needles of sea sponges containing opal. Opuka layer is about 3 - 12 meters thick. It is relatively hard, but it is just above soft permian sandstone. This sandstone is easily eroded and as it is being washed away, the harder opuka layer above it starts breaking and falling down too. This explains the presence of table mountains and deep cuts of valleys in them: Once the erosion finds its way into the opuka layer, it keeps progressing, the valleys grow deeper and wider while the slope continues to be very steep - because the top layer is "fortified" by opuka and resists better the erosion. 

Opuka shaping the towns: There are sevaral areas in Czech R. where opuka has been mined - especially around Prague, in Dzban area (i.e. here), or in Northern and Eastern Bohemia. Opuka as a light and easy to shape rock was a key construction material of Czech medieval cities, incl. Prague. All Romanesque rotundas are built from opuka, as well as the basilica of st. George at Prague castle, or the monastery of st. Anezka the Czech. Opuka was also frequently used by sculptors - let's mention the Celtic head that was found in Msecke Zehrovice not far from here, or coffins of Premyslid kings in Prague Castles's st. Vitus cathedral. Opuka was also used for less noble but equally nice village houses and churches in the villages around Dzban. You must have seen many of them on the way here.

About the cache: The easiest access is from the road connecting Hredle and Treboc. You can leave your car there (see parking waypoint). From the parking follow the path marked by yellow tourist signs. Please note you will be closely following the edge of the plateau. Very often it would take just a few steps among the trees on your left hand side and you would find yourself on top of a very steep slope. From time to time you will get a chance to see the valey down below you. For most of the year the view is obstructed by trees, so if you really want to enjoy it please visit the viewpoint (see the waypoints). It is not far from the final coordinates. Those will bring you near one of the three last active opuka quarrys in Czech Republic. The entry is prohibited, but you can easily see the opuka wall from the road. 

To log the find:

  1. Please send me answers to the following questions via my profile:
    a) What is the origin of opuka?
    b) The word "opuka" can be translated as "cracking stone". What do you think is the origin of the Czech name?
    c) Please find one opuka stone, touch it, feel it. Please describe your findings.
    d) In your opinion, why do the opuka buildings have the stones exposed, i.e. without any plaster? 
  2. Optional: Please attach a photo of yourself in front of opuka quarry.

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