A preferred orientation of rock cracks has been observed on Mars and Earth, which develops after clast deposition. This orientation preference may be caused by solar induced thermal stresses, where cyclical heating and cooling of the rocks due to the planet's rotation introduces the bias, or by differential insolation, where geometric shielding allows some crack orientations to retain moisture and subsequently grow faster than others. Geometric shielding has been examined under Earth conditions, showing preferred orientations of crack propagation, thus it is important to investigate the effect under Martian conditions. Within this work, we use a combination of codes to model the effect of geometric shielding on Mars. The first is a plane parallel radiative transfer code (Doubling and Adding code) and an original geometric python model (GeoCrack). The latter code computes the flux received at the base of a crack of given dimensions and orientation from the results of the former code's atmospheric computations. These calculations are carried out assuming that no reflection of incident radiation from the sides of the crack occurs and shadows are solely due to self-shielding by the crack edges. The flux received by cracks of a variety of orientations and relative dimensions at different latitudes on Mars are calculated, allowing predictions for preferred orientations of crack growth to be made and compared with the orientation preference observed in Martian rocks.