Geoparque Mundial UNESCO
Geoparque Villuercas > Nº32 – Potholes Bohonal
This geosite is located on the northern border of the Geopark on the deep embedment of the River Ibor near where it flows into the River Tajo.
Access is easy on the CC-19 road which connects Bohonal de Ibor with Mesas de Ibor, just below the three bridges of Las Veredas built there at various historical periods to take advantage of the narrowness of the course some 3 km from Bohonal de Ibor.
We will observe one of the most spectacular spots of the River Ibor, a deep defile precisely on the section where the potholes which have been excavated directly in the granite rocks of the riverbed can be found.
The middle course of the River Ibor runs as far as the potholes before it becomes embedded to flow into the Tajo. Since ancient times use has been made of this watercourse to build several river mills, one of which can be found here; we can observe both its stonework and its engineering features, i.e. the waterwheel and channel used to divert the water to the millstone. As far as the landscape is concerned, the old bridge of La Mesta and the potholes, mill, water, and vegetation form a very pleasant aesthetic whole which can be visited particularly in the summer as a place of leisure and bathing.
The embedment of the river has protected the riverside woodland with both old and young examples of alders, black poplars, and willows, which are accompanied on the hillside by Montpellier maples and hackberries sharing the space with other shrubs such as dog rose, which equally seek the presence of cool humid soils.
The geosite is situated some 200 metres downstream of the old bridge of La Mesta and provides a very good explanation of one of the phenomena produced by the river erosion of the hard rocks in the riverbed.
We are here concerned with the geological term of potholes, which refers to large almost spherical concavities caused by the erosive capacity of river water favoured by the discontinuities of the bed and the solid material they drag as they flow over rocky areas. Locally they are known as “pilones” (basins) or “pilancones”.
They form on riverbeds as a result of the revolving action of very hard pebbles dragged by the current which fall into a small depression or concavity at the bottom of the watercourse. When they turn owing to the movement of the water the hard rocky fragments become rounded within these concavities and at the same time the walls of the latter become wider to form the «potholes”.
The hardness of the fragments must be similar to or greater than that of the rocks in which the concavities are formed. This geosite features pebbles of quartzite transported by the River Ibor and the rocks where the cavities are formed are of porphyritic granite (i.e. granite with large crystals of orthoclase feldspar) and highly fractured by orthogonal diaclases (horizontal and vertical fractures). The places where these fractures intersect may constitute the holes which favour the initiation of future potholes. The site is therefore a good example of intensive river erosion caused by whirlpools but there is also a certain tectonic influence (with the presence of diaclases) in its morphology and in its distribution along the watercourse.
