Mudcracks

The marlstone from the Bohlinger Schlucht is formed by a mixture of clay, silt and chalk. This specimen provides important information about the sedimentation process in the northern Alpine foothills during the Middle Miocene. The rock here formed inside a dried-out body of water: most probably a very slow flowing slough or pond, a seasonally flooded floodplain or a lake that had silted up. This can be determined by the tessellated mudcracks. These structures form when muddy sediment dries out. The volume of the sediment shrinks as a result of the loss of water. The surface layer then contracts and sometimes cracks open. If the mud sediment continues to dry out, the individual cracks connect to form a network, as can still be seen on this marlstone slab after 13.5 million years. Such mudcracks continue to form today in areas subject to intermittent flooding or in clay-rich soils that dry out.