[Resumen] In granitic areas the weathering acting on a kind of rock with particular geotechnical properties produce similar or even identical forms in different climatic environments. We shall now refer only to gnarnrnas and tafoni understood as the result of exogene processes,(humectation-desecation, thermical micro-oscillations, solution and others). This great variety of causes to which these microforms can be attributed is interpreted as convergence of processes. Other explanations are based on a common factor to the environments where these forms have appeared; for the gnammas, the rainwater which periodically fills the concavity; for the tafoni a very marked dry season in the clirnate cycle of the area. Others consider them as relict forms born under a different climate and continued in the presente The fact is, however, that none of the aforementioned reasons is simultaneously valid for all the cases. In our opinion, these forms are failure surfaces originated in breaking superficial conditions. Their coincidence with structural surfaces(diaclases)proves that the genesis took place after the delimitation of these surfaces and consequently of the rocky blocks. Let us considera diaclase shaped block, laying on the surface of the ground and submited to its own weigth and supporting reactions, with a resistance to simple cpmpression of 1400 kg/cm2 (equivalent to a non weathered granitic rock). The block's deformation in this case would completely happen in the elastic domain. A change from elastic to failure conditions need an increase in the load acting on the block equivalent to a cover approximately 6 km. thick. However, such depth conditions in the earth crust(implies an increase of geothermic heat and a lithostatic distribution of stresses) inyalidates this model.More shallow conditions are necessary for the blocks to individualize and to allow the circulation of the water along their limiting surfaces. Thus a migration and concentration of charges takes place by means of the rock's differential weathering along the diaclase plains transforming the initially plain surface into an irregular one where the contactbetween the block and the surrounding massif is reduced to a few places. It's there where the load due to the block's weigth plus the cover will be concentrated. Thus, the failure load is much smaller than in our first assumption. requiring a cover thicknesses of only 200 meters. In these circumstances, finite desplacements are generated in certain places of the block which divide the rock initially coherent into units with intergranular or trans granular limits; located in superficial or internal areas of the block, the outter envolving surface of which is called gnarnma or tafoni respectively· Only when the weathering does not destroy totally the structure of the rock massif and if this is exhumated after the u impression " of these failure surface the exogene processes can evacuate the mineral grains which are already predisposed to this separation. This is how both gnarnma and tafoni can be observed as active forms under such different conditions as the detaching mechanisms which expose them. For these reason we cannot confuse the genesis of these forms with the moment in which are manifested regardless of climatic or genetic conditions present or fossil on the sur face of the Earth
