Two factors led to the formation of the Rhine Falls
1. The bedrock. It is much older than the Rhine Falls.
2. The significantly more recent geological processes during the glacial epoch.
The Formation of the Rhine Falls during the Glacial Epoch
As a result of the general termperature fluctuating around 500`000 years ago, the first glacial advances began in the midland and formed the landscape of today.
1. Original course of the Rhine Up to the beginning of the Riss glacial period (approx. 200'000 years ago) the Rhine River flowed from Schaffhausen in a westerly direction through the countryside of Klettgau. This former Rhine valley was then filled up again with alpine debris.
2. First change of the course of the Rhine During the Riss glacial period (approx. 120'000 years ago) the Rhine by Schaffhausen was diverted southwards and formed the Rhine Riss period gorge. Today the basin outlet below the falls is this actual channel, which was filled up again debris.
3. Second change of the course of the Rhine river and formation of the Rhine Falls During the last glacial period, the so-called Würm glacial period, the Rhine was forced to change its route and formed a wide bow towards the south till it reached its present day bed of hard Malm limestone, above the falls. About 15'000 years ago the Rhine Falls were formed when the hard ‘Malm’ limestone at the top changed to the easily eroding debris of the Riss period channel. The Rhine Fall Rocks form the remains of the original steep sloping limestone flanks.
Source: Franz Hofmann, Geologie und Entstehungsgeschichte des Rheinfalls, in: Neujahrsblatt der Naturforschenden Gesellschaft Schaffhausen Nr. 39/1987
Illustration: Typographic Schaffhauser Nachrichten