## 4 Geological Characteristics of Rock Shelters
### Petrology and Geochemistry  
The sandstone host rock of the Tajo de las Figuras site is mainly composed of medium to coarse-grained, poorly sorted sub-arkoses. The framework grains show a relatively high degree of roundness (Figure 3A). It is mainly composed of: quartz (80-90%); feldspar (0-15%), with a predominance of plagioclase; mica (<5%), mainly muscovite; tourmaline (<2%); and opaque minerals (<2%). The matrix is very scarce (<5%) and mainly consists of kaolinite masses (to a lesser extent illite). Syntaxial siliceous cements, ferruginous rims and kaolinite pore-linings and pore-fillings were observed. The chemical composition of the Tajo de la Figuras fresh rock samples (Table 1) is characterised by the high content of SiO\({}_{2}\) (>95%) and low content of Al\({}_{2}\)O\({}_{3}\) (<1.8%) and other elements. In most of the samples, the MgO and MnO content was below the sensitivity limit of the analytical technique employed (XRF). This fact appears to be related to the low proportion of phyllosilicates observed in the petrographic study. Figure 2: Morphological features of the studied rock-shelters (see text for explanation). (**A,B**) Rock Group of Tajo de las Figuras. (**C–F**) Rock Group of Peñas de Cabrera. (**A**) View of the entrance of the Cueva del Tajo de las Figuras, which is located in a fault plane; note the high number of fissures and fractures affecting the host rock. (**B**) General view of the Cueva del Arco. (**C**) View of the interior of the shelter 11; honeycomb weathering is developed on walls and ceiling. (**D**) Lateral view of shelter 13, which is developed on a fault plane. (**E**) Frontal view of shelter 13; decimetre-width tafone are developed in the uppermost part of the shelter wall. (**F**) Frontal view of shelter 17; selective weathering due to proximity to ground level is observed. Figure 3: Thin section photomicrographs. **(A)** Poorly sorted sub-arkose showing clast-supported fabric and grains with high degree of roundness (Cueva del Arco). **(B)** Sub-arkose with feldspar and biotite grains showing fine-grained alteration and pore-filling kaolinite cement (Shelter 13, Pénas de Cabrera). **(C)** Stromatolite-like crust surrounding sandstone surface (Cueva del Tajo de las Figuras). Calcite cements filling intergranular pores and partially replacing both siliceous framework grains and cements. All photographs were taken under cross polarized light. Penas de Cabrera sandstones are very similar to those of Tajo de las Figuras. They are medium to coarse-grained, poorly sorted quartz-rich sandstones with grains showing a moderate to high degree of roundness and sphericity. The sandstone is composed mainly of: quartz (80-90%); mica (5-20%); feldspar (mainly plagioclase) (0-15%); and metamorphic rock fragments (0-10%). The matrix is scarce (<5%) and consists of masses of illites and kaolinites. The cements, although scarce, are mainly siliceous and ferruginous. Most of these sandstones can be classified as sub-arkoses, although locally, sub-lithoarenites are relatively abundant (Figure 3B). The chemical composition is also similar to Tajo de las Figuras sandstones (Table 1). The main difference is clearly the greater concentration of Al\({}_{2}\)O\({}_{3}\) and K\({}_{2}\)O due to the presence of phyllosilicates (mainly mica and illites). The superficial alteration finds on the shelters' walls and of the fallen flakes are formed by a compact mass, which is not very porous and has reddish tones. The chemical analysis for these alteration finds in the two rock groups indicate a marked enrichment in FeO + Fe\({}_{2}\)O\({}_{3}\) (4.9% \(\pm\) 0.5%), MgO (0.42% \(\pm\) 0.17%), and in the loss on ignition (L.O.I.) (1.8% \(\pm\) 0.3%). This fact is consistent with the petrographic observations made and the mineralogical analyses performed. Framework components show weathering features such as: (1) strongly altered feldspars and biotite; (2) masses of authigenic kaolinites; (3) quartz grains with signs of dissolution and syntaxial overgrowth; (4) and muscovites fragmented into flakes.