This valley has a quite linear geographical structure. The only element that "transgresses" this linearity is the so-called Vena del Gesso, a massive chalky bastion that cuts across the valley. It is situated between the villages of Borgo Tossignano and Riolo Terme - Casola Valsenio; to the east it is bounded by river Senio, and to the west it is bounded by river Santerno. Its unique morphology, its sharp cliffs and its wooded slopes mark this chalky formation out of the rest of the landscape. Its vegetation is quite variegated: downey oaks, brooms, poplar trees, wild roses, and other Mediterranean and mountain species can be found here. Also its fauna is diversified, even though not specific.
Since the Vena del Gesso is the only karst area in Romagna, it provides an example for all the phenomena concerning gypsum erosion caused by the course of water streams: caves, wells, dolinas (the only example on the whole Apennine territory) and many more. Unfortunately, only a few of these formations can be reached by visitors, while most of them can be explored just by expert speleologists.
      Throughout the higher and mid Apennine, marly stone and sandstone are the main sedimentary rock formations: these are so friable and soft that they give the landscape a particularly gentle and rounded look.
The main peaks here are Mount Carzolano (1187 meters), where river Senio has its source, and Mount La Faggetta (1144 metres).
The valley of river Senio comprises the villages of Casola Valsenio, Riolo Terme and Brisighella.
Casola Valsenio is especially renowned for its officinal garden, one of the largest in Italy. Nowadays it hosts a leading research centre that studies and manufactures the over 400 different species grown here, even on behalf of a number of universities.
      Riolo Terme and Brisighella are very well-known spas that can easily meet the requirements of any kind of tourism. In fact, both towns as well as Casola, were founded during the Middle Ages and they are rich in historical architectural testimonies. Moreover, due to the excellent environment preservation of their area, these towns can proudly offer visitors their typical products.