The need to analyze clays of the Lower Cambrian and the Upper Vendian as the crackblock structure, taking into account the influence of micro- and makrojointing of rocks in the calculation of stability, is indicated. The influence of sample size of the Upper Vendian clays on the value of cohesion and modulus of total deformation, depending on the level of stress and strain conditions, was analyzed. The influence of microcracks on the mechanical properties of clays is recommended to study under laboratory conditions, makrocracks – through field testing or calculation method.