The character of the Rhön is the result of the Tertiary vulcanism 19 to 25 million years ago, and the following erosion processes. On the base of the oldest rock layers (the Triassic marine deposits of red sandstone, shell limestone and keuper), magmatic eruptions broke through the earth's crust during the early Tertiary, and first formed vents, later basalt layers. On top of these covers, which were preserving the older layers below, marine clay and sand sediments were deposited during the Tertiary. These deposits favoured the development of lignite seams.

During the Quartary, following an uplift of several hundred meters, these new rock layers were eroded away. The only remains are the high plateau around Andenhausen. The basaltic vents and crests were exposed, while at the same time the valleys were filled in with gravel. Consequently, loess was deposited, talus and scree material was formed and upland moors were developed on the inpenetrable rock layers. Dominant rocks are basalt, phonolit, red sandstone and shell limestone, sporadically also keuper.