The Upper Carboniferous Period

 
After the uplifting of a mountain chain preceding the Alps – the so called “Variscian Orogeny”, which also resulted in the last metamorphic imprint of the Silvretta nappe – that high mountain chain was eroded and the sediments were transported by streams and rivers to the plains and the coast. The sediments were deposited as debris, gravel and sand, later also as calcareous and clay layers due to ocean flooding. The rivers were seasonal, dependant on heavy rainfalls. The climate was similar to the situation in Morocco where the Atlas Mountains are located at the Sahara desert today. The climate was dry and hot with rare, short but heavy rainfalls. Along the river banks early types of coniferous trees grew, some of which fossilized and can be found at Bartholomaeberg today. The mica-rich sediments on the right side of the trail are the oldest sediments in Vorarlberg – they mark the base of the marine sedimentary sequence of the northern Calcareous Alps.

If you were wondering why the pebbles of quartz and other especially hard rocks are to a certain extent large and often not rounded but angular, the answer would be because the pebbles originate from the alteration and further erosion of the neighbouring crystalline bedrock. The distance of transportation was rather short and since the transportation of the debris occurred through torrents and mud flows, abrasion and rounding remained minimal.