In Rheinfelden (east of Basel), a salt layer around 50 metres thick can be found at a depth of a hundred metres. It lies beneath water-bearing strata. Although rock salt is highly soluble in water, this salt layer, which is around 240 million years old, has still not been dissolved away by water - even though it is located in an area that experiences earthquakes. Responsible for its preservation are the fifty-metre thick clayey-sulphate layers in which it is embedded. These prevent the infiltration of water. This shows that clay formations have very good isolation properties. (Image: Comet)
