At the beginning of the Palaeogene period of the Cenozoic era (66 to 23.8 million years ago) the formation of the Alps led to considerable tensions in the earth's crust, which, in their forelands to the north of the Alps, caused a bulge in the mantle and crust. At the apex of the arch so formed, there were considerable tensile stresses, so that the rock layers were stretched and about 35 million years ago, deep cracks and depressions in the earth's crust occurred (passive rifting). At the same time the sides of the newly created lowlands were uplifted, in the case of the Palatinate Forest, to a height of about 1000 metres.