On the southwest border of the Duero Basin the Hercynian basement of the Iberian Peninsula is fossilized by a siderolithic sedimentary cover whose thickness increases progressively eastward. Two different palaeoweathering mantles have developed in this zone. From west to east, these mantles are progressively separated in the vertical sense. Whereas the lower mantle affects the Hercynian basement to a depth of > 18–20 m, being fossilized eastward by the siderolithic cover, the upper mantle affects both the Hercynian basement to the west and the siderolithic cover to the east to a depth of c . 3–5 m. Mineralogical, petrographic and geochemical techniques have shown that the lower mantle shows an upward destruction of the parent minerals with the development of new 2 : 1 phyllosilicates (smectite-like minerals) in the middle levels of weathering and an enrichment in 1:1 phyllosilicates in the upper levels. By contrast, the higher weathering mantle shows important palaeosol features, an enrichment in CT opal, oxyhydroxides mobilizations and occasional concentrations of alunite dated at 58–67 Ma. A planation surface appears related to the uppermost of these mantles, its remnants now uneven owing to the Alpine tectonic phases. Eastward, the Tertiary sediments of the Duero Basin unconformably fossilize the remnants of this palaeosurface.

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Some physical properties (bulk and free porosity, pore size distribution), and the chemical composition and mass balance of two deeply weathered profiles one developed on Hercynian granodiorite and the other on pre-Cambrian slates were studied. Hydric and mercury porosimetry, nitrogen adsorption techniques, chemical analyses and XRD techniques were used. On granodiorite, weathering has created increased porosity with a pore diameter \5 lm, whereas on slates the weathering has produced of ca. 1 lm in diameter. These pore sizes have played an important role in the weathering processes. Assuming that weathering preserves volumes, except in the uppermost part of the profiles, it brought about a loss of matter of more than 12% (*300 kg/m 3 ) on granodiorite and ca. 30% (*800 kg/m 3 ) on slates. These changes are related to shifts in the mineralogical evolution, with the appearance of new 2:1 and 1:1 phyllosilicates and Fe oxyhydroxides as the main authigenic minerals. The release of matter, at least since the upper Neogene until the present, has led to the lowering of relief in a more or less homogeneous way, giving rise to gentle hillsides and flat surfaces below which the current river networks are incised.Porosity studies have the potential to explain several specific landforms as well as affecting landscape development in general.

Rañas are alluvial fan deposits of Plio-Pleistocene age that form the piedmont platforms around the mountains in the interior of the Iberian Peninsula. They are composed of cobbles, pebbles and gravels of quartzite and some quartz, all embedded within a clayey matrix displaying striking changes in hues due to hydromorphism. Beneath these platforms, the Hercynian basement is consistently deeply weathered. In the profile of the Raña, the quartzite stones located into the clayey horizon have a weathering rind that is whitish to ochre in colour, in contrast to the dark reddish hue of those located within the leaching horizon, just below the land surface. These differences are related to changes in the physical properties (e.g., bulk density and porosity), mineralogy (presence of oxyhydroxides) and weathering processes that have taken place in the profile. Such processes have led to the corrosion and replacement of the quartz grains by the iron oxyhydroxides. The main cause is the dramatic changes in the water regime occurring in the pores at the surfaces of the quartzite stones. Due to weathering the slates outcropping beneath the Raña have undergone important release of matter (ca. 30%), together with changes in the mineral association, with a progressive reduction in the component of the unweathered slates and an increase in new minerals (smectites, kaolinite and iron oxyhydroxides) upwards.Keywords: porosity, weathering rinds, quartzite stones, slates, piedmont. RESUMENLas Rañas son depósitos de abanicos aluviales del Plio-Pleistoceno que forman plataformas de piedemonte alrededor de las montanas del interior de la Peninsula Iberica. Estan formadas de bloques, cantos y gravas de cuarcita dominantes y algún cuarzo engastados en una matriz arcillosa que muestra importantes contrastes de color causados por hidromorfismo. Bajo estas plataformas, el zocalo hercinico se encuentra profundamente alterado.En un perfil de Rana se distinguen dos tipos de horizontes: i) el superior, de pocos decimetros de grosor, rico en cantos, gravas, arena y limo, pero pobre en arcilla, y ii) el inferior, normalmente de algunos metros de grosor y rico en arcillas. Dependiendo de su situacion dentro del perfil de la Rana, los cantos y gravas de cuarcita situados en el horizonte rico en arcillas presentan una corteza de alteracion de tonos ocres a blancos, en contraste con los situados en horizonte de lavado inmediatamente bajo la superficie, que presentan una corteza de alteracion de intenso color rojo. Estas diferencias están relacionadas con cambios en las propiedades fisicas (p. ej. densidad aparente y porosidad), en la mineralogia (presencia de oxihidroxidos) y en los procesos de alteracion que han ocurrido en el perfil y que han llevado incluso a la corrosion y reemplazo de granos de cuarzo por oxihidroxidos de Fe. La causa principal han sido los cambios hidricos ocurridos en los poros de los cantos y gravas de cuarcita. Las pizarras alteradas que aparecen bajo la Rana presentan una importante pérdida de materia (ca. 30%) y ca...