400 million year old rocks that were folded, pushed over and transformed (metamorphosis) during the formation of the mountains, resulting in their current arrangement and distribution. The Bunte slate formation (a sediment formed in a river plain) and the Hermeskeil formation (a sediment formed in the sea) can be found in the Hintertaunus region. The Taunus quartzite forms the ridges of the Taunus ridge and the rocks of the Kellerskopf formation (a marine sediment), the Rossert metaandesite, the Wiesbaden metarhyolite (both volcanic rocks) and the Bierstadt phyllite (a marine sediment) are found in the Vordertaunus. The Bierstadt phyllite is the oldest rock in Hesse at around 480 million years old. The age of the fine-grained shale, which was only discovered in the subsurface of Wiesbaden in 1991, could be determined on the basis of marine plankton fossil remains. At a very prominent fault zone, the Taunussüdrandverwerfung, the old rock formations have been pushed down to greater depths. It is located in Wiesbaden approximately south of Parkstrasse and is aligned parallel to the Taunus ridge. South of this fault, the rock formations are overlain by Tertiary-age marine sediments. The deepest borehole in Wiesbaden to date, on the site of the former abattoir, encountered the Wiesbaden metarhyolite at a depth of 236 meters. This offset of more than 200 m in the old rock formations indicates that a complicated clod pattern of thrusting and transverse fractures occurred in the course of the Earth's modern history.

The young fractures and fault zones have served and continue to serve as pathways for hot water to rise from great depths in the earth's crust to the surface. The famous Wiesbaden thermal and mineral springs are also the result of the ascent of highly saline thermal deep waters. The steeply rising rock cliffs such as those in Frauenstein and in the forest near Georgenborn were also formed by mineral waters. Thermal waters rose in large fissures, from which first barite and later quartz precipitated, with the quartz replacing the barite in such a way that it took on its crystal form (pseudomorphs from quartz to barite). These fissure fillings of hard pseudomorphosed and capped quartz form the rock cliffs today. The quartz vein from the Neroberg and Schulberg, on the other hand, shows large blocks of quartz. The Schützenhof spring originates here.

A special feature is the basalt volcanism near Wiesbaden-Naurod on the Erbsenacker. Two 57-million-year-old basalt vents were found by geologists. They are interpreted as the channels of a volcano that had formed two volcanic craters surrounded by ash cones and lava flows. The pushing away of the old rock formations of the Taunus along the Taunus margin fault dates back to the formation of the Upper Rhine Graben. As the earth's crust sank within this rift zone, new sedimentation areas were formed, such as the Mainz Basin, into which the sea was repeatedly able to penetrate. This happened partly from the south from the "primordial" Mediterranean Sea, the Tethys, and partly from the north from the Palaeo-North Sea, whose coastline at that time ran approximately at the level of Kassel. The first, short-lived marine advance took place around 33 million years ago and is documented in the subsurface in a 170 m deep borehole near Wallau. The second, widespread and long-lasting marine intrusion occurred around 30 million years ago. The Rupelton (Bodenheim Formation), with its rich, very diverse fossil finds of protozoa (foraminifera), ostracods and fish remains, is evidence of a fully marine sea deposited in somewhat deeper, calm areas. The sea then increasingly retreated and the mica-rich clays and silts of the Cyrenian marl (after the Cyrena fossil of a mussel) show clear influence from the Taunus hinterland, but also a very beautifully preserved mollusc (mollusc) fauna near Igstadt.

At that time, the region around Wiesbaden was roughly on a par with Sicily. The subtropical to tropical climate allowed increasing calcification in the now shallow-marine estuary. This is documented in the so-called limestone tertiary. Limestones and marls with a great wealth of fossils were deposited in a shallow, well-lit lagoon. However, the remaining sea had less and less connection to the oceans of the time. Under these special conditions, algae reefs formed, exposed in the Dyckerhoff quarry, whose early relatives (so-called stromatolites) were the very first fossils on our planet 3.65 billion years ago. Due to the increasing retreat of the sea, the seawater became more and more acidic due to inflows, so that these different living conditions are reflected in the composition of the fossil communities. However, the sediments in the Wiesbaden urban area are also characterized by quartz sand-rich erosion products from the Taunus (lime-free marginal facies), which interlock with the marine fossil-rich sediments. Eventually, the Mainz Basin silted up and only river sediments were deposited. In the course of the Tertiary period, the subsurface of the Northern Upper Rhine Graben and its Tertiary sedimentary overburden sank to a depth of 2000 meters. To the west of the Grünstadt - Nierstein - Hofheim boundary fault, the layers of the Mainz Basin have "got stuck" between the Taunus and Palatinate Forest, so that today they lie relatively close to the surface and can be easily observed, for example, in the Dyckerhoff quarry and in the Weisenau quarry in Mainz (now the Geopfad). Since the late Tertiary and in the Quaternary, today's landscape has been formed: by the uplift of the Taunus Mountains, by valley formation (incision of rivers and streams) and the spread of large river systems of the Rhine and Main in the foreland (subsidence of the southern foreland). Between 800,000 and 500,000 years ago, the Main deposited large quantities of sand and gravel near Wiesbaden, the Mosbach sands. Their rich fossil record is famous. After the Mosbach sands were deposited, the Main deposited a gravel body which, at an average height of 140 m above sea level, characterizes the landscape of the Wiesbaden area with its largely flat terraces. This area extends in the east from Erbenheim almost as far as the Ringkirche church and far to the south via Fort Biehler and from the Second Ring via the Gräselberg to the north of Schierstein. The Rhine and Main then cut further into the former river plain to its present depth.

During the last ice age, the Würm period, storms in the cold season blew out fine material from the vegetation-poor river plains, which was deposited as loess in the lee of elevations. Loess is a calcareous sediment blown by the wind from the gravel beds of large, periodically dry streams. Above the older deposits in the area of the Rheingau field (Mosbach sands), it forms an almost closed, continuous blanket. The Wiesbaden loess profiles generally contain several fossil soils that allow a relative age classification. According to this and to isolated absolute (geophysical) dating, the loesses of Wiesbaden belong predominantly to the younger section of the Pleistocene and there to the last cold period (Weichselian or Würm glacial). Individual bones or teeth of mammoths, wild horses or other large mammals are rarely found in the loess. A camp site of Ice Age horse hunters dating back around 32,000 years, which Thomas Terberger was able to excavate near Wiesbaden-Igstadt, dates back to the time of the loess drift. In the quarry areas of Dyckerhoff GmbH on the Rheingauer Feld to the east of the city, numerous structures of the Ice Age steppe marmot (Marmota bobak) were found in the loess, sometimes still containing skeletal remains of the animals. Due to its composition and structure, the loess has very fertile soils that deserve better protection against overbuilding.