Petrin Rocks EarthCache

Location

Petrin rocks are situated near the top of the Petrin hill close to the top plateau. While walking on the trails in this area you can see numerous outcrops and rocks in all sizes sometimes with caves carved into them. Because of the selective weathering the rocks have different shapes and thus creating specific relief. Unfortunately some of this is covered by graffiti right now. Petrin rocks became part of natural heritage in 1988. The protected area with its size 10,55 ha is situated around Petrin hill and extends to Strahov Monastery and Kinsky Garden. The rocks, formed by sandstones from Late Cretaceous, are mostly covered by forest vegetation.

Geology

This area is protected because of the geological phenomenon – rock outcrops that consist of sandstone from Late Cretaceous placed on the Ordovician slates (from Zahorany and Kraluv Dvur formation). On the top of the isolated outcrops lay sediments from Peruc-Korycany formation (Cenomanian, Cretaceous).

• Stratigraphy: Cenomanian, Paleozoic Era
• Phenomena: discordance, outcrops, transgression
• Genesis: sedimentary
• Mineral: sandstone, argillite, slate

Ordovician period During this period, the Central European area (Mediterranean province) was situated on the Southern Hemisphere around the 50th parallel and formed the periphery of the supercontinent Gondwana. The microcontinent Perunica, situated in the Central Bohemia, has been mostly associated with the faunistic area of Germany, France, Poland and also North Africa.

Mediterranean province was separated from the warm climate areas (Baltica – Scandinavia, Siberia, Laurasia – North America) and colder Avalonia (England, Wales) by Tornquist sea and Rheic ocean. Because of that the possibility of migration of fauna between these areas was limited.

During the Ordovician period, the climate was rather cold. As a proof there is the cold-water Selenopeltis fauna and the dominance of clastic sediments. At first there were some organisms similar to the species that lived in the warmer periods, but from the beginning of the Dobrotiv period the specific Drabovia-Agiromena fauna begun to take over and became distinctive for the Middle Ordovician Perunica.

In the Late Ordovician the migration paths were renewed and thermofilic species from North Europe begun to spread in this area as well. The typical Hirnantian fauna was to be found mostly during Kralodvor and Kosov period and in Barrandien it was often enriched with the elements from Drabovia-Agiromena fauna.

At the end of Ordovician period the climatic conditions were influenced by the glaciations of Gondwana. Because of that the sea level dropped seriously.

Ordovician sediments in Barrandien started to impose in the area of Prague Basin during Lower Tremadocian and continued this process undisturbed till the end of Ordovician period.

Prague Basin itself had a nature of trench with the axes of the SW-NE direction. The width of the Basin was about 15 km at first, but during the Llanvirn and Dobrotiv period it became wider (around 25 km).

Morphology of the bottom of the basin was influenced by a number of sedimentary breaks that were mostly parallel to the axis of the basin and thus were not influencing the linearity at all.

In the central part of the basin there was a significant furrow that was created at the beginning of Arenigian. The Prague shift was pronounced the most in the area between Prague and Beroun. Its northern segment had a decreasing tendency while the south side of the shift had an elevating character.

Between Beroun and Holoubkov the Prague shift was followed by a system of elevation zones. They were mostly active during the Llanvirn and Kosov period. To the west from these elevations the tectonic plates of Rumpal and Bechlov played their role and their surroundings had decreasing character.

The formation of the plates begun during the Llanvirn period in which lagoons separated from the bay by elevation zones were also created. Later in Ordovician (Dobrotiv, Beroun period) these areas deepened and rapid sedimentation followed. In the south part of the basin some elevation zones were more influencing than the others e.g. Prague zone, Mnisek pod Brdy and Hostomice. In the area between Hostomice and Strasice there was a shift that separated the land from the decreasing north part of the shift.

Other significant part of the tectonic activity was the Komarov shift between Strasice and Beroun that worked as a supply channel for the Komarov volcano. This shift was different from the others mostly because of its rotation from the axis (about 20 degrees). And because of that it was proved that this shift follows the breaks in Pribram-Jinec basin from the Cambrian period.
Some of the break lines have retained their activity even during Silurian and Devonian.

The Ordovician sediments are mainly characterized by its clastic development, chemical sediments are only in some of the formations. During the Tremadok period the sediments were mostly clastic, formed in shallow sea with numerous bays. Later, when the center of the basin was sinking intensively, the sediments were mostly clayey (there might have been some sandy sediments on the edges). In the Late Ordovician the bays were deepened again and because of that the sediments were roughly clastic.

The velocity of sedimentation in different formations is also worth mentioning. During Beroun period the sedimentation was very fast and distinctive. Sediments from this period had created powerful sequence of sediments up to 2 km in the central part of the basin.

TASKS THAT YOU MUST FULFILL BEFORE LOGGING FOUND IT:

1. What was the typical fauna for Perunica in the Middle Ordovician?
2. In which period was sedimentation the fastest?
3. What was the cause of the sea level drop?
4. You can enclose photo of you and your GPS.

Feel free to log after sending the answers, if there is something wrong, I’ll contact you later.Send the answers via profile.

Poloha

Prirodni pamatka Petrinske skalky zaujima horni usek svahu Petrina v tesne blizkosti vrcholove plosiny. Pri cestach v horni casti Petrina se nachazeji cetne skalni vychozy, skily a skalky, mnohdy s umelymi jeskynemi. Na jejich povrchu je videt pod spontannimi vytvory nekterych navstevniku Petrina i povrchove tvary a mikrotvary reliefu, vznikajici selektivnim zvetravanim horniny, napriklad vostiny. Skaly a skalky jsou chraneny jako prirodni pamatka od roku 1988. Chranene uzemi ma rozlohu 10,55 ha a zabira horni cast svahu Petrina v blizkosti vrcholove plosiny, pricemz obkruzuje vrchol Petrina od Strahovskeho klastera az k zahrade Kinskych. Skalky, tvorene piskovci svrchni kridy, pokryvaji lesni porosty s prirozenym razem.

Geologie

Predmetem ochrany je geologicky fenomen – vychozy svrchnokridovych piskovcu ulozenych na ordovickych bridlicich.

Izolovane skalni vychozy, v jejichz podlozi lezi sikmo uklonene ordovicke bridlice (zahoranske a kralodvorske souvrstvi), na nich diskordantne spocivaji vodorovne ulozene sedimenty perucko-korycanskeho souvrstvi (cenoman, krida). V ramci regionalniho cleneni patri do Ceskeho masivu - krystalinikum a prevariske paleozoikum - stredoceska oblast (bohemikum) - Barrandien, Cesky masiv - pokryvne utvary a postvariske magmatity - krida Ceskeho masivu - ceska kridova panev - vltavsko-berounsky vyvoj.
 

• Stratigrafie:cenoman, paleozoikum
• Jevy:diskordance, vychoz, transgrese
• Puvod:sedimentarni
• Horniny:piskovec, opuka, bridlice

V obdobi ordoviku se stredoevropsky prostor, Mediterranni provincie, nachazel na jizni polokouli v oblasti kolem 50. rovnobezky. Stredni Evropa tvorila periferii mohutneho prakontinentu Gondwana. Stredocesky prostor, oznacovany take jako mikrokontinent Perunica, byl blizce spjat predevsim s faunistickymi oblastmi Nemecka, Francie, Polska a take Severni Afriky.

Od klimaticky teplych oblasti Baltiky (Skandinavie), Siberie (Zapadni Sibir), Laurasie (Severni Amerika) a chladnejsi Avalonie (Anglie a Wales) byla Mediterranni provincie oddelena Tornquistovym morem a Rheickym oceanem, jenz velmi vyrazne ovlivnoval moznosti migrace fauny mezi chladnovodnimi a teplovodnimi oblastmi.

Klima bylo v prubehu celeho ordoviku pomerne chladne, coz dokazuje krome typicke chladnovodni "selenopeltisove" fauny i prevaha klastickych sedimentu. Zprvu je fauna jeste blizka typum v klimaticky teplejsich oblasti, ale od pocatku dobrotivu dochazi k rozvoji specificke drabovio-aegiromenove fauny, ktera je typicka pro cely stredni ordovik Peruniky. K rozvoji teto fauny doslo predevsim v dusledku rozsirovani a prohlubovani Tornquistova more a nasledne izolace Mediterranni provincie. K obnoveni migracnich cest doslo az ve svrchnim ordoviku. Migrace teplovodnich severoevropskych druhu nastava ve stupnich kralodvor a kosov, coz se projevilo predevsim vyskytem hirnantiove fauny, ktera v Barrandienu obsahuje i hojne elementy puvodni fauny drabovio-aegiromenove. Na konci ordoviku se do klimatickych podminek vyrazne projevilo zaledneni Gondwany, ktere zpusobilo i snizeni morske hladiny.

Sedimenty ordoviku se v Barrandienu zacaly ukladat v sedimentacni prostoru Prazske panve jiz ve spodnim tremadoku a sedimetace nerusene pokracovala az do konce ordoviku.

Vlastni Prazska panev mela prikopovy charakter s osou JZ-SV smeru. Sirka panve byla zpocatku cca 15 km, ale v prubehu llanvirnu a dobrotivu pravdepodobne dosahla sire az 25 km.

Morfologie dna panve byla ovlivnovana radou synsedimentarnich zlomu. Tyto zlomy byly prevazne soubezne s osou panve a tudiz neovlivnovaly jeji linearitu.

V centralni casti byla vyrazna deprese, ktera vznikla jiz na pocatku arenigu a pretrvala az do konce ordoviku. Nejvetsiho vyznamu dosahoval Prazsky zlom v oblasti mezi Prahou a Berounem. Jeho severni segment mel klesajici tendenci, zatimco jizni strana si v nejblizsim okoli zlomu zachovavala elevacni charakter. Vice na jih od Prazskeho zlomu vsak dno rychle prechazelo do hluboke centralni deprese.

Na Prazsky zlom navazoval mezi Berounem a Holoubkovem system elevacnich zon. Ty byly aktivni predevsim v llanvirnu az kosovu. V zapadnim pokracovani techto elevaci se vyrazne projevuji tektonicke kry Rumpalu a Bechlova. Severni okoli techto zlomu melo vyrazne depresni charakter.

Vznik techto ker zacina opet predevsim v llanvirnu, kdy dochazi spise ke vzniku lagun oddelenych od vlastniho zalivu elevacnimi zonami. Pozdeji (dobrotiv, beroun) je patrne prohloubeni techto oblasti spojene s rychlou sedimentaci. V jiznim kridle panve se projevovalo nekolik elevacnich zon v oblasti Prahy, Mnisku pod Brdy a Hostomic. V oblasti mezi Hostomicemi a Strasicemi byl aktivni zlom oddelujici pevninu od klesajici casti severne od tohoto zlomu.

Dalsi vyraznou tektonickou poruchou byl komarovsky zlom mezi Strasicemi a Berounem, ktery fungoval jako privodni kanaly Komarovskeho vulkanu. Tento zlom se od ostatnich odlisoval zejmena v natoceni cca o 20 st. od osy panve, coz dokazuje jeho navaznost na zlomy starsi kambricke panve pribramsko-jinecke.
Nektere zlomove linie si uchovaly aktivitu jeste v prubehu siluru a devonu.

Ordovicke usazeniny se vyznacuji predevsim svym klastickym vyvojem; chemicke sedimenty se nachazeji jen v nekterych souvrstvich. V prubehu tremadoku se vyskytuji prevazne hrube klasticke, pozdeji i chemicke sedimenty, vznikle v melkem mori s cetnymi zalivy. Pozdeji, kdy dochazi k intenzivnimu zahlubovani centra panve, jsou sedimety spise jiloviteho, v pripade okrajovych casti panve pak pisciteho charakteru. V nejvyssim ordoviku opet dochazi k periodickemu zmelcovani a opetovnemu prohlubovani zalivu a v dusledku toho k opet sedimetaci hrube klastickych sedimentu.

Velmi zajimava je i rychlost sedimentace usazenin jednotlivych souvrstvi. Velice vyrazna je velmi rychla sedimentace v prubehu stupne beroun, jehoz sedimenty tvori v centralnich castech panve az 2 km mocny sled usazenin.

Otazky pro uspesne zalogovani FOUND IT:

1. Jak hluboka propast vznikla pri pohybu hornin v roce 1961
2. O kolik metru se posunula restaurace pri sesuvu Petrina?
3. Ktera fauna je typicka pro cely stredni ordovik Peruniky?
4. Zjistete cim se vyznacuji ordovicke usazeniny.
5. Co zpusobilo snizeni morske hladiny?
6. Jako nepovinny ukol muzete prilozit foto Vas a GPS