Höllgrotten Baar

GEOLOGY

PHASE 1 – FROM PREHISTORIC RIVERS

The deeper geological strata in the Zug area are composed of sand and marl – material eroded from the nascent Alps and sluiced to their foothills by prehistoric rivers – and these belts of solid, impermeable rock, known as “molasse”, are covered by layers of sediment and scree that were transported and deposited by glaciers in an earlier period of the Ice Age. This relatively uncompacted – and thus porous – overlaying stratum of gravel formed the base for the ground and lateral moraines generated during subsequent glaciation, and the many kames and endorheic basins between the Sihl and the Lorze are relicts from the last Ice Age; they are also perfect places for large amounts of rainwater to collect and drain away.

The carbonic acid in the rainwater leached calcium carbonate from the calcareous layer of gravel as it percolated down to the molasse; this impermeable layer forced the calcium carbonate-laden rainwater to run off down the walls of the Lorze ravine and – as the pressure dropped and the air forced carbon dioxide out of solution from the springwater – the calcium carbonate was deposited as tufa. Over time, an overhanging curtain was formed, enclosing voids of varying sizes. The entire tufa formation in which the caves were formed belongs to the Holocene, the current geological epoch, and was probably created some 5-10,000 years ago.

PHASE 2 – A STEADY DRIP CAN FORM A STONE

The calcium carbonate deposited by the water droplets conglomerated to create an infinite variety of formations, the best-known of which are stalactites (Greek stalactos = “dripping”) and stalagmites (Greek stalagmos = “dripped”).

Perhaps you’re wondering how these various dripstone formations came about? As calcareous water drips from cracks in the ceiling it releases carbon dioxide, which promotes the formation of seed crystals of calcite; these follow the surface of the waterdrop back to the ceiling, where they adhere, and this is the first step in the growth of a “straw” stalactite. As more water flows down the straw’s exterior, further calcite is deposited and its walls continue to thicken, becoming less fragile and eventually turning into a full stalactite. Water dripping from the end of the stalactite also releases calcite as it hits the ground and this is deposited to form stalagmites, “candle-like” structures which can grow to heights of many metres, although they form more slowly than stalactites.

Geological Monograph

Die Höllgrotten bei Baar, Entstehungsgeschichte und Altersbestimmungen an Quelltuffen (“The Use of Spring Tufa in Determining the Formation History and Age of the Höllgrotten Caves at Baar”; in German), a 20-page monograph published by the Zurich Natural History Society, can be ordered from

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CHF 4.00 per copy, including P&P
Authors: Georg Wyssling, Jost Eikenberg

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