200 million years ago the Jurassic sea extended from the North Sea basin far to the South and covered the late Triassic land. At that time the Hesselberg region was on the border of that sea. Many affluxes brought huge masses of rubble from the eastern mainland and formed a multilayer seafloor, which had a rich flora and fauna. During more than 40 million years the different layers of Jurassic rock consecutively deposited: the Black Jurassic at the bottom, the Brown Jurassic above, and the White Jurassic at the top. Each of these layers is characterised by the typical rock and embedded fossils that are specific for each era. Because certain fossils are solely found in certain layers, they are referred to as index fossils. In Jurassic rock ammonites are the index fossils. In the course of time the Jurassic sea silted up completely. Because during the Early Jurassic Hesselberg was located in a sheltered basin it was not as eroded by wind and water as the plain between the mountain and the Hahnenkamm. The hard rock resisted erosion and left Hesselberg as a distinctive Zeugenberg rising above the landscape like an island. This kind of mountain-formation is known as inverted relief.