Lima is situated on the alluvial fans of the Rimac and Chillon rivers. The palaofans are well exposed along the coastside in the cliffs that can be up to 80 m high. The Cordillera is the supplier of the pebbles and gravels that can be encountered here. Conglomerate clasts can be up to 80cm large at the coast. The Rimac river has a steep gradient of 1:25 and, according to my sources, thus is largly providing bedload transport. Clasts are mostly granites, diorites, gabbros and mesozoic to cenozoic volcanic rocks. The Lima Conglomerate has a thickness of up to 86m. It is interrupted in parts by lenticular sand- and siltstone lenses that likely represent estuarine incursions caused by sea-level variations.
According to what I read they can be subdivided into several layers which, by all honestly, I cannot see. The only difference is the average clast size which perhaps is not well exposed in Miraflores where I am. Neither could I observe the imbrication of the clasts and pebbles indicating flow directions and such alike. Well maybe I did but I have to stretch my imagination a lot to see it. There are no fossils
In terms of age we are talking Plio-Pleistocene here when sedimentation was affected by the movement of the Nazca Ridge towards the south proving for impulses of uplift and subsidence and the Lima anticline that blocks direct access for the Rimac river to the sea.
The pebbles of the Lima Conglomerate get washed out and form the beach. The beach is only pebbles but it makes a great and amazing sound when the waves are washing over them and the water flows back. Klack-klack-klack...
If you like to read more take a look at the cited article.
- Roux, J.P. et al (2000): Sedimentology of the Rimac-Chillon alluvial fan at Lima, Peru, as related to Plio-Pleistocene sea-level changes, glacial cycles and tectonics, Journal of South American Earth Sciences 13, 499 - 510