Geoparque Mundial UNESCO
Geoparque Villuercas > Nº32 – Potholes Bohonal
This geosite is located on the northern boundary of the Geopark, in the Ibor river near its mouth in the river Tagus river.
It is easily accessible from the CC-19 road linking Bohonal de Ibor with Mesas de Ibor, just below the three Las Veredas bridges built there in different historical periods, taking advantage of the narrowing of the riverbed, about 3 km from Bohonal.
We will observe one of the most spectacular places in the valley of the Ibor river, a deep gorge precisely in the section where the “marmitas de gigante” are located, carved directly into the granite rocks of the riverbed.
The middle course of the Ibor river reaches the marmitas before it flows into the Tagus. Since ancient times, this course has been used for the construction of several river mills, one of which is located here, where you can see both its construction and all the associated engineering: weir and canal used to divert the water to the millstone. From a landscape point of view, the old bridge, mill, water and vegetation form an aesthetically pleasing ensemble, especially in summer as a place for leisure and bathing.
The river bed has protected the riparian woodland, with young and adult specimens of alders, poplars and willows, accompanied on the hillside by Montpelier maples and hackberry trees that share the space with other shrubby plants, such as rose hips, which also seek the presence of fresh, damp soil.
The geosite is located some 200 metres downstream from the old Mesteño bridge and explains very well one of the phenomena produced by fluvial erosion in the hard rocks found in riverbeds.
These are the so-called “marmitas de gigante” (giant’s kettles), an expression which, although it has that fanciful nuance, is a geological term that alludes to large, almost spherical concavities, caused by the erosive capacity of the river waters, favoured by the discontinuities of the river bed and by the solid material that they drag along during their transit through rocky areas. Locally they are called “pilones” or “pilancones”.
They are formed in riverbeds by the rotating action of very hard boulders when, dragged by the current, they fall into a small depression or concavity at the bottom of the riverbed. As the hard rocky fragments rotate in these concavities due to the movement of the water, they become worn and rounded, while their walls enlarge in width and depth, forming the ‘marmitas de gigantes’.
The hardness of the fragments must be similar to or greater than that of the rocks in which the concavity is formed. In this geosite, these are quartzite boulders transported by the Ibor river, and the rocks where the cavities are formed are porphyritic granites (granites with large crystals of orthoclase feldspar) highly fractured by orthogonal joints (horizontal and vertical fractures). The places where these fractures intersect may constitute the voids that favour the initiation of future ” marmitas de gigantes”. It is, therefore, a good example of intensive fluvial erosion caused by eddies, but there is also a certain tectonic influence (with the presence of joints) in its morphology and in its distribution along the channel.
