Geological features of the Hundidero-Gato System

            The dam and the area around, such as the Hundidero-Gato system, are of great interest, it is intended that these will be included in a natural and geological reserve by the regional Andalusian government in the near future.

 

            It is situated upon carbonated materials from the sub-Betic or pre-Betic range which are laid upon materials from the Campo de Gibraltar (Alboran) basin.

           Crossing the area, careful observation and some basic knowledge, help us to understand the hydro-geological function of the karstic massifs and, to evaluate the importance of the earlier geological studies done in connection with the water works, and to observe the materials of the two ranges of the Betic Cordillera: the lower Betic range and the Campo de Gibraltar basin.

 

 

 In the area of the Los Caballeros dam, we can see Jurassic limestones and limestone clay (red strata) of the Upper Cretaceous, all these being materials from the interior lower Betic range, constructed as a fallen alticline which has been extensively eroded. The Jurassic limestones form spectacular features of the accentuated relief of the landscape, such as El Hacho and Tavizna.

            These rocks are very fractured and have been affected by intense tectonic activity. This has caused rock falls and spectacular uplift as can be seen when walking on the road past Tavizna.

            The Gibraltar (Alboran) basin occupies the lower parts of the range, having ridden up over the Sub-Betic rocks. There are clays with Eocene intrusions, above which are found pure quartzite sands from the basin.

 

            We can see from the old overflow from the dam, constructed in the 20th century, above the old river bed of the Gaduares, the valley and some of the passes. The bowl of the dam is built on the impermeable rock of the old river bed, made of clay materials from the Gibraltar basin and cretaceous limestone marls from the  Penibetic range. The wall, the first in the world to be built in an arc, is built on Penibetic limestones between Tavizna and Taviznilla, in the ravine which gives access to the Hundidero cave. The shallow depth of the impermeable layer made water-tightness impossible, allowing the dammed water to seep away, to join the water lying beneath the Penibetic limestone commonly found in Karstic regions. The old Electricity Company of Seville never managed to overcome nature although they tried to make the reservoir impermeable using anchored fixings and reinforcing the walls with cement.

       

      Hundidero, through which the river Gaduares flows, is the start of one of the great hydro-geological systems of Andalucia, and has the typical internal caverns produced by the flow of water,  500 litres per second, crossing the Algorrobo-Mures anticline and which reappears at the cave of El Gato, joining the river Guadiaro after a few metres.

 

 The main subterranean system or endokarstic  covers a direct distance of 4.5 km, and is actually 8970m long with a descent of 210m, according to the latest data from the Federacion Andaluza de Espeliologia, who have collected information from the various groups who have mapped the system.