Sheet 173 Elm of the Geological Atlas of Switzerland (1:25,000 series)
The Elm geological sheet includes part of the Tektonikarena Sardona World Heritage Site surrounding this Alpine peak in eastern Switzerland, which features the Glarus thrust and other tectonic highlights in starkly glaciated relief. The most spectacular profile is displayed along the Tschingelhörner ridge, to the southwest of the Piz Segnas mountain (3099 m), where the darker Permian Verrucano, including volcanic rocks, were pushed northwards over younger folded and thrusted sediments. The profile comes complete with the Martinsloch, a ~20-m-wide hole formed along the intersection of two faults that displace the Glarus thrust – at certain times of year the Sun shines through this aperture to illuminate the church in Elm before rising above the ridge. The ridge is illustrated from both sides in two panoramic profiles; alongside a tectonic map, and fabulous series of six highly complex and detailed grid-referenced natural scale 1:25,000 cross sections, printed on separate sheets of paper. Beneath each photograph, cartoons name the tectonic boundaries without labelling the coloured units, so by mistake the Tschingelhörner-Schuppen underlying the Glarus thrust has been confused with the Aar-Massiv in the key.
The geological map itself is drafted to an incredibly high standard, as part of an ongoing surveying and publication programme, so the download can be enlarged to 1:10,000 scale without any loss of detail. Enlargement shows a variety of late glacial and Holocene superficial deposits and features, including geomorphological delineations and stippling, alongside the exposed bedrock labelled with a logical series of abbreviations. Symbols are used for structural information and to identify points of interest. Other linear features, including faults and thrusts, are identified using different colours, which become dashes, or dots and dashes, beneath areas blanketed by superficial deposits.
The memoir in German also comes as a pdf document allowing for portions of text to be copied and loosely translated online. This booklet provides a thoroughly referenced account of the different sequences caught up in these nappe complexes, as well as their structures, often intense overprinting foliation and low-grade metamorphic history. The text is beautifully illustrated in colour with plenty of clearly drafted diagrams and maps. Excellent well-captioned photographs, including partly coloured in and annotated panoramas, highlight structural features of interest. The brief Quaternary section encompasses a synthetic image analysis of land slipping alongside aerial photography for the same area, and historic rock falls, including a massive slate mining collapse in 1881 that flowed past Elm covering ~80 Ha mapped as a unit.
Reviewed by David Nowell
BY: Brok, B. den, Caduff, R. & Kempf, O. (2021) Blatt 1174 Elm. Geologischer Atlas der Schweiz 1:25,000, Karte 173. Federal Office of Topography, swisstopo, CH-3084 Wabern, Bern. 208 pp. (pbk), 3 plates and geological map ISBN 978 3 302 40110 2
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Nowell, D.A.G. (2021) Potential of swisstopo and Swiss Geological Survey websites for Quaternary researchers. Quaternary Newsletter, No. 154 October, 27-51
Extract from geological map 172 Elm, 1:25,000 topographic sheet 1174 covering area around the Tschingelhörner ridge and Martinsloch. Copyright © swisstopo, All rights reserved.