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Geology and Geomorfology [ Geomorfologie ]

The Krkonoše Mountains and several adjacent ranges of the Sudetes form the northern fringe of the Bohemian Massif, a wedge of the European Hercynids protruding between the Fenno-Sarmatian platform and the Alpine-Carpathian system. The Krkonoše-Jizerske Mts. complex of crystalline metamorphic rocks (mica schists, phyllites, and orthogneisses) of Proterozoic to Palaeozoic age (100 to 400 million years) forms much of the Krkonoše Mountains. This crystalline complex was folded twice in the Palaeozoic and, during the second period of folding in the Carboniferous, was intruded by a granite pluton that forms most of the main ridge and almost the whole of the Polish side of the mountains. Around the pluton is a hard contact zone; this has been of great importance for the mountains’ geomorphological evolution, and this is where the major ore deposits are found. Quartzites, basalt, and crystalline limestones also occur in the region and, despite their limited extent, are important in determining relief and vegetation structure.

In the Mesozoic and the first half of the Tertiary, when tropical and subtropical climates predominated in Central Europe, chemical weathering remodelled Krkonoše into flat and smoothed ”grading surfaces”. Subsequently, the mountains were intermittently and slowly uplifted during the Upper Tertiary by orographic movements, related to folding in the neighbouring Alpine and Carpathian systems. While they thus attained their prominent height through uplifting, this was followed by rapid water erosion and the dissection of the grading surfaces into today’s broken mountain relief.

Another important geomorphological factor was mountain glaciation during the Quaternary Ice Ages, when the relief was modelled both by alpine glaciers (the origin of glacial cirques, lakes, moraines, etc.), and by frost processes (the origin of cryoplanation terraces, frost cliffs and screes, polygonal soils, etc.). These forms are the most valuable abiotic features of the national park and biosphere reserve. In the Postglacial era, the main remodelling factors have been water erosion, landslides, and anthropogenic influences. Frost and biological weathering are less pronounced and proceed together with avalanche erosion and, in the limestone area, with karstic processes.

Geological Evolution
The Giant Mts., together with Jizerské hory belong, from the geologic point of view, to one complex called krkonošsko-jizerské crystallinicum. Crystalline schists (mica schists, phyllites, quartzites and crystalline limestones) of the archeozoic or Paleozoic age, (i.e. about one billion years old) prevail in it. The orographic processes of the Palaeozoic age were significant for the origin of the Giant Mts. when they, after the last siluric sea flood, about 400 million years ago, became dry land. In that time so called caledonian folding occured which determed the main features of Giant Mts. crystallinicum. During the later variscian (hercynic) folding a very significant process occurred when into the core of cystalline schists granite magma arose and created the so called Giant Mts. granite massif (Giant Mts. granite pluton) to a thickness of about 5 kilometers. The granite hardened in the depths, and it reached the surface of the mountains only later by grinding off the upper layers of rock. The main part of the Polish Giant Mts. and the entire Hraniční hřbet from Harrachov to Sněžka is built up by the granite massif. The Krkonoše granite is pervaded by a system of cracks which condition its characteristic ashlarshape decomposition (well seen on the edge of Labské jámy (pits) or on the top rocks, so called tors), unlike the crystalline schists by which mainly the inner Český hřbet and the adjacent mountain saddle bows are made. The rocks break into smaller sharp edged fragments, so while the granite Hraniční hřbet is a rather round shape, the mountain parts being built up by the crystalline schists have a more sheer shape (e.g. Kozí hřbety, Obří hřeben (Giant Ridge)).

The glowing granite mass in contact with the old crystalline schists changed the structure of the surrounding rocks and caused origination of new minerals (e.g. hornstone). At the same time many crude ore deposits arose in the contact zone that were intensively exploited in the past and also today they have become famous mineralogic localities (Mísečky, Svatý Petr (Saint Peter), Obří důl (Giant Vale) etc). For detailed information about the geologic structure of the Giant Mts. see the picture).

During the Mesozoic and the older Tertiary periods there were no significant orographic or tectonic processes, nevertheless the warm and damp tropical to subtropical climate conditioned an especially chemical and mineral weathering. A thick weathered mantle denuded by water and wind was shaped and the weathering affected ever deeper layers. The height differences were reduced and the surface of the Giant Mts. got only a mild undulating character of so called etchplén. The originally hidden granite core was successively uncovered, mostly on the main ridge, and the typical loaf shaped rocks, by specialists called tors or in the local terminology stones (Mužské and Dívčí kameny (Men's and Girl's Stones), Harachovy kameny (Harrach's Stones), Polední kámen (Noon Stone) started to arise.

Only toward the end of the older Tertiary other significant changes took place in the Giant Mts. and that was in connection with the alpine folding during which the Alps and the Carpathians originated. Powerful pressure invoked by the folding of the Alpine-Carpathian arch also made the area of the Giant Mts. higher, almost to today's level. It revived the river erosion and the following quickly changing shape of the mountains. The river modelling resulted in deep and narrow valleys and the sharply broken mountain relief, depending upon the geologic bedrock and the different hardness of the individual rocks. Because the retrogressive erosion did not reach the main river streams, the original Tertiary surfaces level, the so called etchplains (Labská, Pančavská and Navorská louka (meadow) in the western and Bílá louka (White Meadow) with Úpské rašeliniště (peatbog) in the eastern Giant Mts., were preserved in their surroundings. Toward the end of the Tertiary periods the rivers as well as the shape of the mountain range, broken into ridges and saddle bows, were almost similar to their present shapes and in the later periods only the valleys deepened. The Polish part of the mountains has developed easily, with respect to the uniform granite bedrock and the short sheer slopes.

In the older Quaternary period the climate in all of Europe got cooler and the ice age started. Though the huge continental ice sheet came repeatedly (at least twice) near the northern foothills of the Giant Mts., the Mountains themselves have never been covered by ice continuously. However, the cool climate supported the origin of the local glaciers of the Alpine type. Together there were 11 with different long tongues (the longest glaciers in Labský and Obří důl were almost 5 km long), and more than two dozen other small glaciers in the ends of the glacial valleys so called cirques. The glaciers changed the narrow mountain valleys to become glacial troughes having the characteristic U-shape in their cross section. On valley bottoms and sides the glaciers accumulated many terminal and lateral moraines composed of assorted stony till, clayey material which had been drifted and put up on various places by the glaciers. The best-preserved moraines are in Labský and Obří důl and in the lower part of Kotelní jámy. On the Polish side in the cirques of Wielki and Maly Staw and in Sněžné jámy there arose some glacial lakes and tarns enclosed by moraines ving. Strong frost weather on mountain ridges resulted in the origin of other specific processes and shapes (frost cliffs, periglacial screes, cryoplanation terraces, and polygonal grounds). Thus, the ice and postice ages have left an uncommonly varied group of glacial and cryogenic (frost) phenomena the range of which is very unique in the midpart of Europe. By rights they make up the most valuable parts of the nature of Krkonoše National Park.

Later in the milder climate, especially water erosion but also various types of processes of declination found their use again. Debris avalanches, so called land slides formed by strong overwetting and the following motion of the slope weathering after strong summer and torential rains occurred very often. In the Giant Mts. about 180 trails of debris avalanches have been recorded, primarily in the area of Obří and Dlouhý důl. Snow avalanches which are described separately, are also a significant factor in the Giant Mts.

The important product of the Quaternary period are peatbogs on the ridges, the area of which reaches almost 300 ha. They came to originate in the wet and cool Atlantic period, during the last ten thousand years when the accumulation of big quantities of rain water in the flat depressions supported the growth of peat moss and other hydrophilous plants. Their withered parts changed gradually, due to the bad air access, into large layers of peat. The peatbogs on the ridges of the Giant Mts. have a significant Nordic tundra character and have played an important role for the local flora and fauna.

The layers of the crystalline limestones and dolomites at the foothills of the Giant Mts. caused the origin of various karst processes, especially in the southern and eastern part of the Mountains. The karst phenomenon is represented, besides sporadic clints (grikes), and flarks or bog pools, by small, pure decorated caves (except Bozkovské jeskyně situated near the Giant Mts.). The biggest ones can be found in Rokytnice, Poniklá, Štěpanická Lhota, Maršov, and Albeřice where the longest measures almost 200 meters.




LAST MODIFY: Jana Holáňová (Beskydy-Valašsko, regionální agentura CR) org. 2, 10.09.2004 v 14:46 hodin

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