## 6 Discussion and Conclusions  
The results of field work and the laboratory analyses showed that the most common mechanisms of alteration in the study area are: carbo-hydrolysis (chemical weathering), frost weathering (mechanical weathering) and bio-induced encrustation (biotic weathering). Other mechanisms such as haloclasty, abrasion, or slaking are considered to be of lesser importance. Haloclasty, combined with other mechanisms, has been interpreted as being  
Figure 7: Human and animal-induced deterioration. (**A**) Sandstone excavation near the top of shelters 11 and 13; the floors of these artificial depressions commonly dam water, and the development of organic soils is favoured. (**B**) Wasp nests on the ceiling of the Cueva del Tajo de las Figuras. the most important in coastal environments ([33; 34; 41; 65], among others) and even desert areas [66]. However, in our case study, it is not easy to evoke a source of airborne salts in the form of an aerosol [67; 68] because of the long distance to marine waters and industrial centres [69]. The very scarce gypsum crystals in the alteration rind and in the bio-induced crust are due to interstitial fluids which migrate outwards controlled by environmental conditions [67; 70]. This is consistent with the existence of levels of gypsum in the underlying facies below sandstones beds [20; 25]. Hydro-aeolian abrasion processes participate mainly in the removal of weathering debris in tafoni and alveoli [14; 19; 40]. Both wind-blown and water-driven deposits, including organic matter remains and salts, partially cover the shelter surfaces and/or infiltrate through the porous host rock. The shaking phenomena must have more influence on some shelters (e.g., shelter 17), where repeated cycles of wetting and drying facilitate extensive flaking by means of ordered-water molecular pressure mechanisms [47; 71]. The similar structural and lithological characteristics of the studied sandstones mean that, qualitatively, the alteration processes that affect them are similar. In this sense, this has only been observed as the presence of a greater proportion of slaty grains in the Penas de Cabrera sandstones and their separation by thermal weathering processes causes the formation of lines of small alveoli which was not observed in Tajo de las Figuras. However, it was proved that the different climatic conditions in the location of both rock groups directly influence the intensity of physicochemical weathering of sandstones. The Tajo de las Figuras area shows a rainfall index three times higher than in the Penas de Cabrera area. In addition, the Tajo de las Figuras area has a greater seasonal contrast, with a wider thermal oscillation range and frequent frosts during the winter months. These differences in the climatic features cause the recognised alteration processes and the interaction between them to be more effective in the Tajo de las Figuras rock group, since:  
* The greater availability of water leads to greater intensity in the chemical alteration processes. The most obvious effects of which are: (1) a higher degree of alteration in the feldspars, with kaolinite neoformation; (2) a greater development in case hardening processes with the formation of superficial alteration rinds, which are enriched in silica, Al and Fe, and Mg and Mn oxides;
* This hardened rind, a priori, inhibits the development of mechanical erosion processes but favours the retention of the water infiltrated in the new rock-rind contact area. When cracking or flaking occurs, the existence of a discontinuous rind produces differences in the distribution of permeability, humidity, etc., favouring the alteration processes. This is particularly noticeable in the discontinuity planes which constitute the contact areas between the superficial alteration rind and the underlying rock. The latter is generally bleached, relatively poor in cations, such as Si, Al, Fe, and Mn, and is crumbly.