 |
| A lovely view of the Rhine-Main basin from the 10th hole of Rhineblick Golf Course (Wiesbaden, Germany) |
The story of our region begins in the
early Cenozoic era. One may recognize this era name for other grand
events; the Cretaceous–Paleogene mass extinction and the
accompanying rise in the fortunes of our mammalian ancestors, but it
was also an era of continuing great geologic movements. As with the
Indian plate’s movement northward, which we discussed in class, in
this period the African plate was moving north against the Eurasian
plate as well. This pressure threw up layers of metamorphosed
sedimentary rock, which had previously for ages been ocean floor, in
the form of the Pyrenees and the Alps. On the other side of these growing mountain ranges, the collision resulted in a substantial
thinning of the lithosphere, isolated volcanism, and the formation of
a rift system known as the European Cenozoic Rift System (ECRIS).
 |
| A digital elevation map of the Upper Rhine Graben and a side view of how it was formed |
Like the more famous Great Rift Valley
in Africa, the Rhine rift features a central linear depression
(running on a north-south axis) through which the river runs; this is
known as a graben (from the German word for ditch, coined by
German geologists in the 19th century when deciphering the
geology of this region). This subsidence caused by the thinning of
the lithosphere was also accentuated in some areas, the Vosges
Mountains and the Black Forest for example, by uplifts on the rift
flanks in excess of 2,500 meters. More recent glaciation throughout
the Pleistocene add the finishing touch to the craving up of the
landscape.
 |
| The ECRIS and the associated grabens and faults |
The Rhine-Main is not only a junction
of rivers, but also of faults; the Upper Rhine Graben meets the Lower
Rhine Graben (which channels the river on via the Low Countries to the North
Sea) and the inactive Hessian Grabens. This area has been the constant recipient of the worn down gravels
and sands of the mountains and upper tributaries, creating a fertile
plain in the basin 40-50 km in width.
 |
| Artist's impression of the Basel quake |
The violent
tectonic origins of the region have not only left their mark in
geologic record, but human history as well. The Kaiserstuhl
(Emperor's Chair) north of Freiburg is a cluster of old, now extinct
volcanoes from the Miocene which in time became covered with a 10-40
meter layer of loess, clastic wind-blown silt. This has
provided farmers with rich, well drained soil for intensive
agriculture for hundreds of years; erosion being kept in check by the extensive use of terraces. Beneath the plain is one of the largest aquifers in
Europe, holding an estimated 450,000 km3 of fresh water. 3 million
people in Germany and France depend on this supply for three quarters
of their drinking water. On the less positive side, seismic activity
is not a thing of the past. During the early Middle Ages (c. 800 –
1000), chroniclers often made note of full earthquakes and tremors,
which they reckoned was punishment for the sins of the people or
monarch. The most destructive earthquake to occur in the region was
the Basel earthquake in 1356, which flattened the city, drained
lakes, and collapsed buildings up to 200 km away.
 |
| A view of the plain at the city of Mannheim (seen on the horizon) |
As you can see, the evolution of the
landscape has not just made pretty vistas for us to look upon; it
continues to shape the life of people today even in regions of the world
that you might never have considered "geologically active." We will dig more into this as we continue to explore the story of the Rhine-Main.
