Geoscientist Online

Richard Fortey reports on a new geological attraction in Portugal, complete with trilobites, gold and jumping nodules…

Geoscientist 18.12 December 2008

At the 33rd International Geological Congress in Oslo in August a session was devoted to proposals for Geoparks. These are UNESCO approved areas of significant earth science interest, which wrap up geological spectacle with appropriate development for local communities. The concept has sparked considerable interest everywhere from Albania to Brazil, by way of Kazakhstan and Malaysia, and it does provide a way of tempting tourists and scientists alike into little-visited regions in need of an economic fillip. Geoparks should be good for geology, and benefit the regional economies. Small wonder that many of the proposals come from countries that could use an infusion of tourist dollars.

In June 2008, I was fortunate to visit a proposed Geopark near Arouca in northern Portugal: the park of the giant trilobites. The presence of trilobites in the Ordovician Valongo Formation had been known about for half a century¹ but only recently has a quarry in a hard black shale used for roofing slates extensively revealed the layers containing the giant trilobites. Thanks to enlightened management of the exposures on the hillside near the village of Canelas, the bedding planes which are covered with these giant animals have been carefully preserved. Many of them have been displayed in a trilobite Museum built for the purpose. There is nowhere else in the world where one can see walls literally covered with half-metre long trilobites, belonging to perhaps a dozen species. For the invertebrate fossil-lover it is difficult to imagine a more thrilling exhibit.

The Museum has been well set out with panels explaining how the trilobites occur, and explaining something about the biology of these extinct arthropods. A rather wonderful and huge reconstruction of Calymene legs and all demonstrates the morphology. One of the large beasts is even named after the author of this article.

Most of the trilobites are enlarged versions of related trilobites that might be found in outcrop in Wales or Bohemia, for example, as parts of Ordovician Gondwana. Not all the species were capable of reaching enormous size – that prize seems to belong to the asaphid and lichid trilobites in particular. Since they often occur as entire specimens rather than as moulted, sloughed-off fragments, it is likely that these animals were victims of an occasional mass mortality, possibly while they were assembling to mate. The mid-Ordovician sedimentary basin which extended across Iberia may have been liable to brief bouts of anoxia. It may be that the largest trilobite that ever lived will be discovered among this fauna, one which may approach one metre in length, although a small measure of tectonic distortion influences measurements and must be carefully taken into account².

The compressed shale lithology of the Valongo Formation does not preserve much in the way of trilobite tracks, but this deficiency is countered by abundantly preserved Cruziana trace fossils which can be inspected nearby on bedding planes of the underlying Santa Justa Formation. These are very like those from the Armorican Quatrtzite, and were probably made by the same species of early Ordovician age.

The same quartzites hosted gold-bearing veins that were mined out by the Romans. A slightly scary path takes the visitor down into a dark cave lined with ferns that was excavated along the now exhausted lode. The pools where heavy gold particles were concentrated by slave labourers two thousand years ago can still be inspected at the surface. The combination of giant trilobites with tracks probably made by some of their close relatives, and the evidence of historical human mining, serves well to suggest to visitors the compass of geological time, and its human consequences.

Nor is this all. Variscan granites nearby offer spectacular scenery, particularly notable waterfalls where sedimentary rocks abut the intrusion. Aficionados of such things can inspect superb examples of ‘corn bread weathering’ on nearby granite tors. One relatively small intrusion is apparently unique. The pale Catanheira Granite is remarkable for including abundant black discoidal ‘nodules’ of biotite up to about 12cm in diameter. They are sufficiently striking to be visible from a passing car. During the hot summers that regularly toast the region these nodules pop out of the rock spontaneously. This has given rise to the local legend of “the stones that give birth.” Presumably because of ‘sympathetic magic’ the nodules have acquired the reputation for assisting conception for couples who are having problems in having children. The type locality of the granite with the pedras parideiras is now protected from over-enthusiastic collection of these curious nodules.

In detail, the nodules have a ‘core’ of felspar which is surrounded by a thick layer of biotite. According to Reavy et al.³ the spontaneous ejection of the nodules is a result of the differential dilatation during hot weather. Once the nodule starts to move the biotite cleavage lubricates its sudden ejection from the matrix.

These geological and palaeontological wonders will be both protected and explained within the Arouca Geopark. The Portuguese Ministry of the Environment, Territory and Regional Development are important sponsors. In the town centre of Arouca a monument has been erected in the middle of a roundabout, ornamented with – naturally enough – giant trilobites. Much of the impetus for the project comes from local geologists and palaeontologists. Of the latter, Artur Sá arranged the visit for the trilobite experts from the Fourth International Symposium on trilobites that was held subsequently in Toledo. He has been indefatigable in advancing the case for Geopark status for the area. The visiting scientists were even treated to trilobite cookies. It is about time the trilobites had a show to rival the dinosaurs.

References

1. Thadeau, D 1956. Note sur le silurien Ibeiro-durien. Boletim da Sociedade Geologica de Portugal 12, 1-38.
2. Sá, AA & Gutierrez Marco JC (eds). 2006. Trilobites gigantes das ardosias de Canelas (Arouca). Ardosias Valerio & Figuerido, 1-207.
3. Reavy, RJ, Hutton, DHW & Finch, AA 1993. The nodular granite of Castanheira, north central Portugal: origin of the nodules and evidence for diapiric mobilization of granite. Geological Magazine, 130(2), 145-153.