Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)

Stone tables, their predispositions and genesis

 

Jan Lenart

Geoscience Research Reports 50, 2017, pages 255–261

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Published online: 31 October 2017

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Abstract

The paper describes the predispositions and the genesis of stone tables, which are naturally - not anthropogenically - formed, rather rarely occurring geomorphic rock forms of table-like shape. They are composed of subhorizontal desk overlying the cavity and held up by at least one supporting rock block. Part of the table or even the entire table is not in its original in situ position, because individual components or the table could have been gravitationally shifted. For closer investigation, 8 stone tables from 3 geomorphologically different areas were chosen (Fig. 1).
Five stone tables occur in the Radhošťská hornatina Upland, which is a part of the Outer Western Carpathians, composed of flysch rocks of Cretaceous-Paleogene age. One locality is situated below the highest peak of the Ondřejník geomorphic unit, which is considered to be a nappe outlier consisting of Cretaceous flysch rocks. Two remaining tables occur in the Šatovská pahorkatina geomorphic unit as a part of the Bohemian Massif. The area is formed of Proterozoic orthogneiss. Directly on rock surfaces of the tables, the structural measurements were performed using geological compass. The results were presented as rose and pole diagrams and were compared with the reference measurements from the surroundings. Where there was a lack of proper rock outcrops, the structural measurements from crevice-type caves situated close to the individual stone tables were used. The results are shown in Figs 2 and 3. Except for table “c”, all tables are formed in close connection with the development of slope deformations. Table “c” is formed within a forest road cut in the gravitationally non-affected area. The results indicated that the basic predisposition for stone table formation is the exposure of subhorizontal sedimentary beds, metamorphic layers or similarly jointed magmatic rock in relation to the dip of slope. The second predisposition is a vertical jointing of the mature rock. The genesis of stone tables is connected mainly with gravitational processes, when individual blocks are shifted and joints become widened, or with the selective weathering and erosion, when particular components of stone tables are shaped. However, the stone table is very often a result of combined complex processes, when the slope is totally affected by gravitational processes with subsequent effect of selective weathering and erosion. Unlike dolmens described in literature, the anthropogenic origin of stone tables described in this paper is doubtful. The evidences against the artificial origin are as follows: (i) great amount of detected forms in different phases of evolution, (ii) tight similarity between structural values measured on the tables and reference values from the surrounding rocks or crevice-type caves, (iii) identified context, when tables or their components are always formed in vertically jointed rocks, which are gravitationally affected, (iv) inaccessibility of the localities, often within the landslide rocky terrains.