Climbing to the summit of Gamila (2,497 m) on Mount Tymfi, at an altitude of about 2,100 meters, one encounters strange spherical structures along and around the trail. If you look up to catch your breath, you will see the same structures, even larger in size, embedded in the limestone masses. Within the gray, massive limestones, spherical to elliptical stone masses protrude, with concentric zones of multiple layers inside them. Shapes and forms reminiscent of tree rings, fossilized giant snails, the eyes of some exotic creature, or even… the eggs of the dragon that is said to have once lived in Tymfi and fought with the dragon of Smolikas.
These impressive rock formations are known as the “large spherical concretions” of Tymfi, a rare and unique geological formation characteristic of the UNESCO Vikos-Aoos Global Geopark. Although they have long been known to mountaineers in the area, only recently has a team of geologists begun to study them systematically. The impetus came in 2022, when Hungarian geography professor Tamás Telbisz (Eötvös Loránd University, ELTE) was mountaineering in the Gamila area and wondered how they were formed. Similar formations can be found in other parts of the world, but their presence in Tymfi is special: not only do they contribute to our understanding of the geological history of the northern Pindos, but they also shed light on a lesser-known aspect of the traditional architecture of Papigo in Zagori. Thus, these geological features emerge as crucial links between the geodiversity, geo-heritage, and cultural landscape of Zagori.
The concretions appear as large stone spheres, ranging in diameter from a few centimeters to over a meter. They are spherical or slightly elongated, with distinct layer alternations inside them, visible along fractured surfaces. They were originally located on the rocks around the path to the summit of Gamila, but the action of water, through erosion and dissolution of the limestone, led to their gradual detachment and rolling down towards the path. In some places, these spheres can still be seen arranged in rows, embedded in layers of limestone, forming sections that extend over a length of more than 20 meters.
The formation of these stone crusts is related to the process of ‘diagenesis’: the set of physicochemical changes that lead to the conversion of sediments into solid rocks over geological time. These chemical changes occur when marine sediments rich in biogenic material (mainly shells of microorganisms rich in calcium carbonate, CaCO₃), are deposited on the sea floor and begin to fossilize. If, at that moment, water rich in silicon penetrates the pores and organic fragments, the process of silicification is triggered, i.e., the precipitation of quartz and, in some cases, the replacement of carbonate material with silicate.
The reason for the development of concentric shells, rather than irregular shapes, remains partly unclear. It may be related to small-scale deformation of the material when covered by newer sediments, as well as the fact that calcium and silicon do not easily mix to form a mineral together, but instead form separate calcite (CaCO₃) and quartz (SiO₂). These concentric shells, like ancient tomograms, record the processes of their formation—processes that were never completed, as the limestone formations of Tymfi emerged from the sea about 15 million years ago. If the diagenesis had been completed, the silica would have migrated, forming nodules or layers of flint elsewhere: the dark, glassy-looking rock with a conchoidal fracture that we often find in the limestone layers in Tymfi and in the ravines of Vikos and Aoos.
Even more fascinating, however, is the microcosm of these rocks. Observation with a petrographic microscope reveals a whole world of microfossils. Nummulites dominate, protozoa with a disc-shaped form reminiscent of tiny coins (their name comes from the Latin nummulus, “small coin”). At the same time, species such as Alveolina and Orthophragmines are also found. Micropaleontological studies have shown that these organisms, which belong to the group of “large benthic foraminifera,” lived in shallow tropical seas about 45 million years ago and, after their death, were carried by sea currents and deposited in deeper environments, where they formed the basis for the creation of concretions.
The presence of spherical concretions is not limited to the highlands of Tymfi. A smaller occurrence has been recorded on the rocks of the northern base of the stone arch bridge of Noutsos (or Kokkoris), near the village of Kipoi. If one descends from the top of Gamila towards Megalo Papigo, one will see the same material carved and incorporated into the facades of traditional houses from the 18th and 19th centuries. This presence was first identified by Mr. Haritakis Papaioannou, Dr. of Biology and coordinator of the Vikos-Aoos Geopark, during the recording and study of stone carvings in houses and traditional buildings in Megalo and Mikro Papigo. These apotropaic stone carvings (hologlyphs), which depict stern human faces and dragon heads, were used to ward off the “evil eye” and their use was once widespread in the traditional architecture of Zagori and Epirus.
This example from Megalo Papigo demonstrates that geocultural heritage is not confined to the subsoil or to the maps of specialists. It has penetrated culture, popular imagination, and everyday life. The use of spherical comparisons between local limestone and the mortars of another era is proof that the mountain is not just a place for hiking or climbing. It is a book—a stone book that is read slowly, word by word, layer by layer, at every altitude.
In the Vikos-Aoos Geopark, therefore, the promotion of these geological formations takes on particular significance. Not only because they are natural monuments of significant scientific value, but also because they bear the imprint of human memory, traditional art, and the unwritten cultural geography of the place.
Bibliography:
Chatzipetros, A., Stergiou, C., 2016, Morphotectonic indications of uplift from the Vikos Gorge area using UAV: Preliminary results, In: Kranis, H., (Eds.), Abstracts volume of the 1st TSG Meeting, Geological Society of Greece, Athens, December 6, 2016, (in Greek with English abstract).
Papaioannou, H., Chatzipetros, A., Kitsaki, G., Stergiou, C.L., 2023, Advancing geoheritage education in Vikos-Aoos UNESCO Global Geopark: A review of the 1st Summer School, European Geoparks Magazine, 20, 68.
Stergiou, C.L., Papaioannou, H., Dimou, G.V., Peristeridou, E., Melfou, M., Stamatiadis, A.G., Melfos, V., Koukousioura, O., Kitsaki, G., 2025, Sedimentary, Metamorphic, and Igneous Rocks in Harmony: Unveiling the rocky microcosm of the Vikos-Aoos UNESCO Global Geopark, Abstract volume of the 17th International Congress of the Geological Society of Greece, Bulletin of the Geological Society of Greece, Special Publication, Extended abstract 300, pp. 1396-1401.
Stergiou, C.L., Chatzipetros, A., Paschos, P., Nikolaou, E., Papaioannou, H., 2021, The Vikos-Aoos Geopark in Greece: Recent Advances in Geoscientific Research, Abstract volume of the DIGITAL 9th International Conference on UNESCO Global Geoparks, Jeju Island UNESCO Global Geopark, Republic of Korea, December 12-16.
Telbisz, T., Stergiou, C.L., Mindszenty, A., Chatzipetros, A. 2019. Karst features and related social processes in the region of the Vikos gorge and Tymphi mountain (Northern Pindos National Park, Greece). Acta Carsologica 48(1), 29-42.
Tsoupi, M. 2006. Epirotic Stone Carvings: Art and Society – Tracing in Zagori, 1st ediiton. Gefyra publications.
