Former valley of the stream Anoia now abandoned and filled up

The river Calders is a series of meanders in a embedded valley in the light grey, marine rock, mainly of coral source, from period Eocene. In its stretch in Viladecavalls of Calders, in front of the cliff of the fountain Calda, there is an unexpected lateral change from Eocene rock to continental, reddish sediment from the Quaternary. The colours emphasize the discordant contact. Reddish sediments lay horizontally in its centre as a result of a calm settlement, but in slope near to the Eocene rock, where they look colluvial debris.

These reddish, continental and modern sediments that are placed between the hard marine rock must come from an ancient river valley, nowadays sealed. Indeed, we are facing a specific river capture phenomenon, in which same river that loses one tributary captures it from another point. The reddish sediments relate to an ancient mouth of the stream Anoia that, once upon a time, joined the river Calders coming in perpendicular from north, from the area of ?? El Canadell. But hundreds of thousands of years ago, the stream of Anoia was caught by same river Calders, but 2 kilometres downstream, near the ridge of the hydroelectric station Jorba, after rounding off another meander. The retrogressive erosion of a small stream that used to have its mouth here achieved to catch and redirect the bigger stream Anoia. The two mouths, former and present, are not that far; the stream Anoia just changes its north-south direction in its last 500 meters when suddenly turns west because the capture. The phenomenon was possible because the river Calders flows in meanders. Despite filled in with sediments that the river Calders laid down at the time it flowed at higher level, the small valley left by the stream Anoia still displays some of its old relief.

The abandonment and capture of stream of Anoia one meander beyond by the river Calders is another element of geological heritage of Calders valley, in addition to the oxbow or cut off meander of the Castle, its long succession of meanders, the lime stones from reef source and its marine fossils.

[photos Jordi Badia]