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.
Conglomerate from the Spanish Pyrenees
These clasts have not seen much transportation. Very little rounding off as in a normal conglomerate. this looks like a breccia.
Yes, it is possible that ‘breccia’ in this case may be more appropriate. These rocks are definitely not very good examples of typical conglomerate. However, even slight rounding is rounding. Isn’t it? And somewhat rounded these clasts are. As much as I know there is no defined roundness that must be achieved to call it conglomerate. These things are largely a matter of preference. These two rocks may just accidentally be more angular than average. I believe they are called conglomerates in this region. I was part of a geology students group visiting this area. Our professors called this rock conglomerate. So it isn’t just my interpretation. I believe they took it from the literature describing that area.