Fluvial terraces are the areas where rivers or streams deposited gravel in recent geological times. The depth of the river terraces is normally 1 or 2 meters, until roughly ten meters the deepest ones in the district of Bages. The altitude of the river terrace indicates the level of the water flow some tens or hundreds of thousands of years ago. In several stretches of Cardener and Llobregat valleys, nowadays a series of stepped river terraces display alternating periods of predominance of either river sedimentation or erosion.
Gravels are detrital, non-cemented sediments made up of over 2 cm diameter stones. Although with exceptions, in the districts of Bages and Moianès these stones are usually round as a result of its transport by rivers. They are usually included in a sandy and clayey matrix. They were laid during quaternary period, therefore they’re the youngest geological materials in these districts. The gravels are also common in the riverbeds.
Many deposits of gravels have been quarried.
The 1st image displays the fluvial terrace ten meters above the current riverbed of the Llobregat in Balsareny. The slightly tilt strata of reddish continental Eocene rocks are in discordant contact with the flat fluvial terrace above. The 2nd image, obtained in Sant Salvador de Guardiola, repeats the same arrangement at smaller scale with gravel from the stream of Guardiola.
[photos Florenci Vallès (1st) and Jordi Badia (2nd)]
- See also the article Manresa geosites, in Catalan, by Jordi Badia.