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Ertsafzettingen, Geodynamics and Geofluids, Geologie

Les Blancs Cailloux


Chapel at Ucimont, completely built with massive quartz boulders.

West of the small village of Mousny, located some 10 km south of La Roche-en-Ardenne, Belgium, a peculiar massive quartz occurrence can be found at the locality called “Les Blancs Cailloux”. The legend goes that these isolated massive quartz bodies are nothing less than a malignant shepherd and his herd turned into stone, when the cold-hearted shepherd repeatedly refused water and food to a pilgrim, actually being Jesus of Nazareth in disguise, on his way to the hermitage of Saint-Thibaut.

A detailed petrographical, microstructural and mineralogical study of the vein quartz and a geochemical analysis of the fluid inclusions has revealed that this Mousny massive quartz occurrence has being formed in a dilational jog, comprised within an extensional shear zone related to the late-orogenic extensional collapse of the slate belt.

All over the High-Ardenne region, huge quartz boulders are used as decoration in gardens and as building material for chapels and houses. We suggest that the widespread presence of these boulders indicates that late-orogenic massive quartz occurrences, as exposed at Mousny, are common in the very poorly exposed High-Ardenne slate belt.

The results of this study are now published in Geologica Belgica: Depoorter, S., Jacques, D., Piessens, K., Muchez, P. & Sintubin, M. 2014. The Mousny massive quartz occurrence – the vestige of a late-orogenic dilational jog in the High-Ardenne slate belt (Belgium). Geological Belgica 17(3-4), 293-310 – URL:


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