The geology of the region around Barcelona is extremely complicated. Recently, geologists have been investigating the composition of the landscape ahead of a new underground train line construction. Their job was a difficult one because existing maps were not detailed enough for the purpose, there was not enough published data, most of the area is now covered in urban development, and rock outcrops are relatively rare in the study area. To map the area at the required scale, they depended heavily on earlier unpublished geological fieldwork, with new surveys undertaken by themselves, added to the results of boreholes drilled at key points, and examination of exposed rock faces at the locations of the two new stations.
One of the locations that revealed evidence for the geological composition and structure near to the proposed underground rail line, was the famous Parc Guell on the northern edge of the city. Here, the winding pathways around the park have often been carved into the hillside, exposing vertical sections of colourful stratified rocks. I photographed some of these exposed rock sections because I thought the combination of earth colours, the patterns of parallel layers, the networks of cracks and crevices, and the overall textures were attractive. To me, each image represents a natural abstract work of art. I could imagine the blocks of stone as intriguing stand-alone sculptures or wall-mounted slabs resembling thickly-applied oil paintings.
I can’t say exactly what type of rocks are illustrated by each photograph. The stratigraphy of the area is exceedingly complex. I can say that in the Parc Guell location various rock types are represented. All of them are sedimentary rocks laid down over an extensive period of time – around 150 million years. The rocks range from the Silurian deposits of dark shales and black chert (laid down from 439 – 409 million years ago); to Silurian and Devonian deposits of limestones and shales (439 – 353 million years ago); through to some Carboniferous sandstones, shales, conglomerates, black chert and limestone (353 – 290 million years ago). I think my pictures show mostly limestones and shales of Silurian to Devonian date and are part of what is called the Pre-Variscan sequence.
There is a plethora of variously coloured rock types in the Barcelona hills around Parc Guell. The series of rock types of the Silurian period in this region includes quartzitic white slates, quartzites, black shales, white siliceous shales, iron oxide rich shales, and black chert. The Devonian series of strata includes massive nodular limestone, pink shales, yellow limestones alternating with red shales, marls and nodular limestones, and brown-green shales. While the Carboniferous period is characterised by dark shales with black chert layers, pink-yellow limestones and dolomites, shales, conglomerates and sandstones.
The colours seen at the surface of the outcrops of rock can be altered by transformation of the minerals they contain through exposure and weathering. For example, Silurian black shales are sulphur-rich mainly due to high levels of pyrites (iron sulphates and iron hydroxides); and these weather out as multi-coloured red, brown and yellow streaks. The black shales also contain smaller amounts of calcium, magnesium and aluminium sulphates that turn white with weathering.
Some of the patterns of lines in the rocks are evidence of the original bedding when the sediments were first deposited. Other strange networks of cracks, crevices and fissures result from pressure or violent disturbances in the earth’s crust. There were massive earth movements known as the Variscan Orogeny shortly after these rocks had been formed. This was a geologic mountain-building event caused by the late Palaeozoic continental collision between Euramerica (Laurussia) and Gondwana to form the super-continent Pangea – a plate tectonic process that was a continuation of the Caledonian Orogeny of the Silurian period.
These earth movements resulted in great alterations to the layers of sedimentary rock – with bending and folding, compression, ripping, cracking and tearing, uplifting and downward displacements, changes caused by heat and friction (metamorphism), and intrusions by volcanic rocks (igneous) . This area has the remnants of many geophysical features such as anticlines, synclines, normal faults, reverse faults, and half-grabens.
Click on the link below if you would like to learn more about the geology of the hills around Barcelona. Most of the information for this short blog was gleaned from the recently published research by P. Santanach (Departament de Geodinàmica i Geofisica, Universitat de Barcelona) and his colleagues.
COPYRIGHT JESSICA WINDER 2013
All Rights Reserved
Jessica's Nature Blog 