Middle Jurassic - the Alb and its feet of clay
Next to his two „brothers“, the older Lower Jurassic and the younger Upper Jurassic whose fossil finds have made them world-famous, the inconspicuous „little brother“, Middle Jurassic, might easily be forgotten. And yet he has quite a lot to offer.
The Middle Jurassic stretches as a relatively thin band along the northern side of the Swabian Alb, between the flat Lower Jurassic plain and the steep rise of the Upper Jurassic. Its brownish, mainly ferruginous sandstone gave it the distinguishing name of Brown Jurassic, in contrast to the dark oil shales of the Lower Jurassic and the white compacted limestones of the Upper Jurassic. Compared to the Lower or Upper Jurassic, the Middle Jurassic layers were deposited around 160 million years ago in a shallower sea which was nearer to the mainland and in particular much cooler. At a water temperature of only 13 to 18°C, it was simply too cold for corals and other tropical creatures. Nevertheless, a number of mussels and ammonites have been found, some of which have opalescent shells.
At the time of the Middle Jurassic period, the area of the eastern Alb was nearer to the mainland than the western Alb. The different conditions under which sediments were deposited can still be seen in the landscape today. A first cuesta in the area around the central Alb was formed by the Donzdorf sandstones which used to be in great demand for building stones e.g. for the construction of the Ulm Minster. These sandstones are among those contributing to the formation of the so-called ferruginous sandstone. Iron ore deposits can be found here, washed out of the rivers of the neighbouring mainland. As these layers have an iron content of over 30%, they had been an interesting mining source. For example, about 30,000 tons of ore a month was excavated from the „Karl“ mine near Geislingen up to 1963.
Robert Gradmann described the Alb very aptly as a „colossus with feet of clay“ − for the massive steep slope of the Alb actually lies on the Ornatenton, the uppermost Middle Jurassic member which is prone to landslips. This is where part of the water from the karstified Upper Jurassic comes to the surface in the form of springs and due to its spring erosion, contributes significantly to the periodic landslides of parts of the Alb. When observing the landscape, it is usually quite easy to see the border between the Middle Jurassic (picturesque hills in the Alb foreland) and the Upper Jurassic (craggy Alb rise). We can also see the Middle Jurassic in the extensive meadow orchards at the edge of the Alb as this has always been the main use of the land.
continue with Upper Jurassic