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Geological Map of the Aar Massif, Tavetsch and Gotthard Nappes

Aar MassifThis 1:100,000 map presents a clear overview of the bedrock forming the Swiss central Alps, from the Doldenhorn, Eiger, Titlis and Tödi mountains along its northwest flank, to beyond the upper Rhône and the main headwaters of the upper Rhine in the southeast.    

Along strike from SW to NE, where it swings round more towards the east, the map spans 110 km, covering a swath of rugged terrain 30 to 40 km wide and clearly showing the principal groupings subdivided into over 100 colour coordinated units. These are identified by letter codes which could be larger within stippled units, and lower-case letters often obscured by the intricate cartography of the excellent base map, even if the keys are easy to read.   

The bedrock is subdivided along strike by a number of very distinct Nappes, Variscan thrusts, faults and thrusts, plotted under the main glaciers and Quaternary fill which is only shown in the main valleys. In addition, four beautifully drawn true scale cross sections are shown separately on another piece of paper, cutting across the entire mountain range to just below sea level. Ideally these would include the sense of displacement along the major faults which they depict. Two of them include the Lötschberg and Gotthard Base tunnels – 2,450 m below the surface beneath Piz Vatgire.

The accompanying booklet outlines the geology of the crystalline basement and other units in brief detail to form the majority of the text, before an informative and reasonably long account of the area’s tectonic and metamorphic evolution. Where possible, the results of this work harmonise historic lithostratigraphic and tectonic units, so they are tabulated alongside their new equivalents into a more coherent synthesis based on international principles. This forms a key part of the Swiss geological data model which will become the basis for future surveys, to be published as new sheets in the 1:25,000 Geological Atlas of Switzerland series.   

Naturally the geological evolution of the Alps since Ordovician times is highly complex, so the text could have been enhanced by some paleogeographic reconstructions.  However, there are plenty of high quality photographs, often clearly annotated, plus many neatly drawn colour diagrams throughout the text. In particular, the map showing the facies concept with an insert diagram showing their temperature and pressure distribution is very clear, along with a P-T evolution graph for selected localities. 

Given its size, the map would have benefited from back folding to make it easier to handle, rather than four panels dropping down 86 cm with a 126 cm width. Also, even if its depth had been increased to 100 cm (used for the 1:625,000 Bedrock Geology maps of the UK), it would not have been possible to fit easily beneath the map the series of cross sections shown separately on another piece of paper. 

Finally, a brief section outlines some open questions which have yet to be resolved: the Vättis Window; the junction between the Aar Massif, Tavetsch Nappe, Glarus Nappe Complex and Ilanz zone which could fit one of three interpretations; and subdivision of the Aar Massif.      

By David Nowell

GEOLOGICAL MAP OF THE AAR MASSIF, TAVETSCH AND GOTTHARD NAPPES by BERGER, A., MERCOLLI, I., HERWEGH, M. and GNOS, E.  2017.   Geological Special Map 129 with explanatory notes, 1:100,000.  Federal Office of Topography, swisstopo, CH-3084 Wabern, Bern, 126pp (pbk) ISBN 978 3 302 40093 8  List Price 20 CHf