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Geology is the Way

Urban geology: the nummulite fossils of the stone walls of Girona

Girona is a lovely fortified medieval town in Catalonia, in the northeastern part of Spain. The city was built at the confluence of four rivers by the ancient Iberian tribe of the Ausetani with the name of Gerunda, later becoming a roman fort and, finally, a small town in the Early Middle Ages. As many other settlements in the Iberian Peninsula, this city witnessed the struggle between Christian Kingdoms and the Moors during the Reconquista. Girona was captured and sacked several times between the AD 715 and 982. Many buildings in the cities were first built by the Moors, then repurposed by the French and the Aragonese. An example is the 14th century gothic cathedral that dominates the city, which stands on the site of a pre-existing mosque, that was reconsacrated in AD 908.

Girona cathedral

The cathedral of Girona dominates the city in this view of the Riu Onyar.

With this turbulent past, it is no wonder that the city is surrounded by a series of fortifications, towers, and walls, which were built over pre-existing Roman defense systems in the 14th century and later adsorbed within the city. In the 19th century part of the fortifications were demolished and  abandoned. Just recent restorations works made the walls accessible to the public again.

Girona walls

Sections of the stone walls of Girona still encircle parts of the old historic center. The larger section lies to the south and the east of the city.

The city is not interesting just for its history, good food, or the beautiful overviews of the historical center of the city from the city walls. While I was walking on the elevated passageways between a tower and the next one, some limestone stones nested in the crenellation captured my curiosity.

Pedra de Girona block

A block of Pedra de Girona: a nummulitic limestone that formed in tropical reefs and lagoons more than 40 million years ago!

The walls are made of blocks of the Pedra de Girona, a building stone that is used everywhere in Catalonia. In Girona, many Medieval buildings, including the neighboring cathedral, are built using this stone. The Pedra de Girona (stone of Girona) is an Eocene (Lutetian: 48 – 41 million years old) nummulitic limestone that is widespread in the area. All the rounded objects included in the stone are fossils of nummulites.

Nummulitic limestone Nummulitic limestone Nummulitic limestone

Most of the limestone blocks are packstones, where nummulites and other large foraminifera (like Alveolina and Discocyclina) are packed together, surrounded by calcarenitic matrix. Nummulites are benthic foraminifera that lived in the Mediterranean Region between the Eocene and the Miocene, at the bottom of warm, tropical seas.

Benthic foraminifera build lenticular calcite shells with a long internal spiral constituted by a regular sequence of many, tiny, chambers that are screwed in a complex way in the 3-dimensions. They may appear tiny creatures (these ones range from some millimeters to 3-4 centimeters), but you should consider that they are single-celled organisms!

Nummulitic limestoneNummulitic limestone

In some cases, fossils are well preserved enough to see the external shape of the shell. As shown above, benthic foraminifera have a very rough shell, with granules and trace of the septa visible. In this case, where the surface is broken, we can have a glance of the internal spiral.

When the foraminifera is cut in half, we can admire the beautiful, internal spiral structure:

NummuliteNummulitic limestone

There is something more to see here. If you play with the slider above, you might notice that the shells are merged together, sutured along some wriggly contacts. At the same time, many cracks crosscut the fossils. This is an effect of lithification (i.e. the transformation of loose sediment to rock), and possibly also of deformation. When the foraminifera-bearing lime sediment is buried, it is subject to stronger pressure and higher temperature due to the load above: fossils are squeezed together, fracture, and may start to dissolve. Chemical dissolution acts where two shells are in contact and gets pressed against each other. This phenomenon is called ‘pressure solution‘.

Here are other pics from this ‘urban outcrop’ for you to enjoy!

Nummulitic limestone Nummulitic limestone Nummulitic limestone Nummulitic limestone Nummulitic limestone

There are also other objects packed together with these nummulites. Can they be fragments of other fossils, like bivalves?

Bonus question for the Paleontology-sleuths following me: Even if these are generally called nummulitic limestones, they contain many other foraminifera like alveolina and discocyclina. Can you identify all the species visible in these snapshots?

Further Reading

GEOFLAIX – Pedra de Girona (Catalan)
Papazzoni & Sirotti (1995). Nummulite biostratigraphy at the Middle/Upper Eocene boundary in the Northern Mediterranean area. Rivista Italiana di Paleontologia e Stratigrafia.
Racey (2001). A review of Eocene nummulite accumulations. J. Petr. Geol.
World Foraminifera Database.
A “Muralla” in Girona by Scott Engering.
Footage of a living foraminifera by microscopeitaly.
Larger benthic foraminifera on GW.

     

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