Overview and images of conglomerate as a rock type are here: Conglomerate
These beautiful stones from the Spanish Pyrenees may have formed in several different ways. One possibility is an ancient riverbed. However, the angularity of individual stones (clasts) makes this assumption rather unlikely.
It is easy to notice that riverbeds of mountain streams are usually composed of rounded stones. Another major way how these rocks form are gravity driven debris flows on flanks of mountain ranges where alluvial fans are formed.
The conglomerate contains two major components – gray limestone and reddish sandstone. Therefore we need both of these rocks to be exposed nearby to feed an alluvial fan with a coarse grained material. Nearly vertical mountain walls of versatile composition are pretty good candidates for the provenance for such a rock type.
The rock is few hundred million years old. So it was formed well before the Pyrenees mountain range emerged. Pyrenees are a relatively recent phenomenon (geologically speaking of course) which were formed approximately 50…25 million years ago. These rocks have had an opportunity to witness and take part in the slow formation of the mountain range.