There are 2 types of conglomerate rocks in the massif of Sant Llorenç del Munt i L'Obac according to the composition. Main conglomerate is polymictic, so made of different rolling stones types. It contains stones from the paleozoic era (granite, quartz and metamorphic rocks) and also limestones and sandstones from more recent periods. Polymictic conglomerate are not so strong cemented by a sand, reddish matrix. Into the polymictic conglomerate, there are 4 strata, from few to 20 meters thick, of limestone conglomerate, this is the specific type of oligomictic conglomerate that contains almost only round limestone stones. They're white colored by the carbonated stones and more consistent. Different composition of the conglomerate results into different erosion pattern. Polymictic conglomerate can be eroded and disassembled producing large, round areas. On the other hand, limestone conglomerate which are strongly cemented results in small, vertical cliffs. On the contrary, all conglomerates in Montserrat are polymictic, so without the stronger and whiter layers of limestone conglomerate that are found in Sant Llorenç del Munt i L'Obac.

The picture and the drawing display the layers of limestone conglomerate in the landscape of the north face of Montcau and Cortins mountains, in natural park of Sant Llorenç del Munt i L'Obac. See also the display of the limestone conglomerate in the range of L'Obac. [photo Jordi Badia, drawing Oriol Oms]

See the article Conglomerat (= Conglomerate), in Catalan, by Jordi Badia.