The nummulitic limestones are made of cemented skeletons or shells of nummulites. Both, the nummulites and the cement are calcareous nature. Nummulitic limestones are characteristic from the Eocene epoch, in the Tertiary period.
The nummulitic limestones deserve one close look. The lentil shaped fossils of nummulites are seen. The diametrally cut nummulites display their beautiful structure of geometrical, attached cells. The nummulites (Nummulites sp.(†)) are protozoa of the Foraminifera group – thus unicellular organisms. They grew massively in the Eocene seas, but they became extinct later on. The nummulites secrete an external skeleton of calcium carbonate -like corals and molluscs- in lentil shape, usually 5-8 mm diameter, although there are individuals reaching 25 or 30 mm that are understood as an alternation of generations in their life cycle. They use to live in sea plankton. Once nummulites died, the skeletons deposited at the sea bottom. The accumulation of shells of nummulites and their further cementation has sourced the strata of nummulitic limestones that are the most widespread sedimentary rock made by living organisms in Earth. The whole nummulitic limestone is one lumachela, one rock made of fossils. A cut and polished nummulitic limestone displays the delicate internal arrangement of the nummulites. In addition to limestones, nummulites can also be found free when disaggregated by erosion.
Nummulitic limestone strata are useful to correlate amongst stratigraphic columns from different locations including marine Eocene. In the Bages district, nummulites and nummulitic limestone strata were studied by the geologist Valentí Masachs.
The rock from the quarries of Sant Vicenç de Castellet that is famous for strength and beauty and really appreciated in construction is one nummulitic limestone. The layer on top of Tavertet cliff, in the district of Osona, is one nummulitic limestone as well. Back to the district of Bages, the natural bridge of Les Arnaules, in Manresa, is made of nummulitic limestone. The 1st image comes from a rock outcrop in Manresa, near El Pont Vell; the 2nd image comes from one stone at the entrance of the secondary school Lluís de Peguera, also in Manresa.
[photos Jordi Badia]
- See the article “Pedres milionàries” (= Million-year rocks), in Catalan, by Jordi Badia