Formation

When the African and European tectonic plates collided during the Tertiary period (65–2.6 million years ago), the European crust was exposed to immense tectonic stress. In southern Europe, this led to the formation of young mountain chains the so-called "alpide belt". But these forces also affected central Europe.

A vast rift system was formed, which extends from the North Sea to the Upper Rhine Rift and the Rhone Rift in southern France. The graben structure of the Upper Rhine Rift is still clearly visible in the landscape today.

The Mainz Basin is thought to be an area adjacent to the northwest flank of the Upper Rhine Rift that had not subsided as much as the actual graben. Between the Eocene (around 55 million years ago) and the lower Miocene (around 20 million years ago), the formation of the graben structure created a sedimentary basin with lakes, which was filled with clay, silt, sand and marls from the surrounding highlands.

The basin as a marginal sea

In times of high global sea levels, seawater flowed into our region and turned the Upper Rhine Rift Valley into a narrow seawater channel and the Mainz Basin into a lagoon-like bay.

The marine environment is evidenced in the rich fossil record, which includes seawater fish, sharks, corals, marine snails and clams. Sea cows also lived in the sub-tropical waters. Several fossil sea cows, including a complete skeleton were found in the clay pits of Flörsheim in 1902 (to be seen in the Natural History Museum in Mainz).