At the beginning of the Devonian about 416 MBP a flat sea extended the ocean Tethys to the south of a line from Cologne to Hannover. Temporarily an island string was lifted out this sea at the todays Hunsrueck and Taunus. This is proved by fossil Brachiopoda and Coral. From here you can see on top of the Hunsrueck, Taunus and Rochusberg the early Devonian rocks, which are classified into colored slate, gray phylite, Hermeskeil sandstone, Taunus quartzite and Hunsrueck slate. Hermeskeiler sandstone and the superposed Taunus quartzite were built of sand, that has been deposited near the coastline. These hard rocks are todays highest elevations around. The following depression caused their overlay by clay, which has been compressed to Hunsrueck slate. During the late Devonian and early Carboniferous about 360 MBP these mountains had been folded and long period of mainland began here.

During the following millions of years the Rhenish Slate Mountains stayed mainland and erosion processes dominated. Mesozoic sediments have not been proved yet here.

The substratum of the Mainz Basin is built from rocks that originate from the Tertiary. 38 to 12 MBP the area of todays Rheinhessen was covered by a sea. This tertiary sea basin was about 50m deep and is named Mainz Basin. It was the bay of an arm of the sea that connected the ancient North Sea ? a part of the opening North Atlantic Ocean ? with the Para-Tethys ? a part of the closing Tethys ? in Paleogene. The formation of this sea happened in two phases intermitted by a siltation.

38 to 25MBP the Oligocene sea and its coastline were similar to todays Caribbean. There were subtropical temperatures, the mean annual temperature was ca. 18°C (today 10,5°C).

On the sandy bay grew palm and bay trees. The water was populated by crocodiles, ocean turtles, sea cows, sharks, skates and a multitude of fishes. The land was populated by flamingos, pelicans, rhinos, apes and elephants.

Rupel formation and later with upcoming brackish water Cyrene marl were deposit on the sea bed and near the coastline sands. During the Late Oligocene the Mainz Basin was almost land.

25 to 12 MBP the Mainz Basin was part of a depression and a second sea phase took part. Unlike the sand and clay deposits during the Oligocene era lime sediments were then deposited. Todays plateaus and hills are built by them.

These strata are denoted from their type fossil, in chronology at first Cerithium stratum, then Corbicula stratum and at last Hydrobia lime stratum (Hydrobia elongata).

In all these Tertiary strata are up to 250m elevated. The border between Devonian and Tertiary rocks is located near the line Ruedesheim ? Marienthal ? northern Johannisberg. It marks the coastline of the Mainz Basin against the Rhenish Slate Mountains. Composite and bedding of the strata denote a cliff line, that is still there noticeable, where the ancient surge broke against the Taunus quartzite.

The land phase of Rheinhessen began when the sea withdraw 11 to 3 MBP. Since then the Rhenish Slate Mountains elevated about 240m. Erosion processes predominate with the exception of the Rhine Graben, that is still depressing and therefor an accumulation area.

In Pliocene the todays Rhine, Main and Nahe river systems arose. They meandered through a flat rippled landscape.

The sandy strata of this period are denoted Dinotherium sands. The type fossil is the elephant like Dinotherium giganteum, that achieved a shoulder height of 5m.

From the beginning of Quaternary the temperature sunk noticeably. Cold periods with permafrost and warm periods with climates like today, partly almost warmer, altered. The glacial rivers cutted the lime strata into plateaus and so the basic shape of todays Rheinhessen landscape arose.

During the glacial cycles the Rhine deposited gravel to terraces. The oldest terraces (Main Terrace) are located near the Taunus edge, the middle and lower terraces are attached to these down to the Rhine like stairs. The sands and gravel are covered by Loess deposits in varying thickness. Apart from rests of the former glacial cycles todays Loess deposits arose mostly during the Wuerm glacial.

The quaternary Rhine was dam up the Rhenish Slate Mountains. Therefor the relatively soft tertiary rocks were eroded in the bottom of the wide Rhine valley. The Rhine flowed branched in multiple channels through the softer parts of the bedrock. The tougher components remained as islands, the channels were filled with sand and gravel.

Tafeln des Geoökologischen Lehrpfades Gau-Algesheim
Rathaus, Marktplatz 1, D-55435 Gau-Algesheim.

Wikipedia, Stichwörter Mainzer Becken und Rheinhessisches Hügelland.

DLR - Dienstleistungszentrum Ländlicher Raum: Böden der Weinbergslagen Rheinhessens.