There are several reverse faults in folded area of district of Bages, like the one in the picture from Vilafruns (Balsareny). The arrows display the relative movement of the blocks. On the right, there is one tilted anticline fold.
Reverse faults in depth often evolves to anticline folds on surface; likely this is the case of this small fold in Vilafruns.
Above the folded strata that were deposited in Oligocene (32 million years ago), there is a fluvial terrace made of alluviums from river Llobregat, roughly from 25000 years ago. At this time, the riverbed was 15 meters upper than the current one. The fluvial terrace is made of a layer of gravel down and a layer mainly of silt on top. This terrace lays on continental rocks from Oligocene period, which are mainly sandstones down and, on top, mainly mudstones containing thin layers of limestones in between. The sandstones and the mudstones came from sand and mud that settled in a temporary lake of quiet water in rainy episodes.
The contact between the Quaternary fluvial terrace and the Oligocene is a slight slope, that’s locally waved due to erosion, towards the present river.
[photo Florenci Vallès]