In the White Jurassic period, about 161 million to 150 million years ago, a shallow sea lay across the whole of southern Germany. During this period, thick rock sequences were deposited on the seabed as the earth's crust constantly sank. The landscape of Franconian Switzerland is strongly influenced by the resulting limestone and dolomite rocks of the White Jurassic. It is a typical karst landscape with deeply incised river valleys and dry, arid plateaus. Many fossils, especially ammonites, may be found in the limestone strata.