The landfilling of inert material, such as rocks and soil, has been lower-rated as it is considered unlikely to cause land, water or air pollution.
Mae tirlenwi deunydd anadweithiol megis creigiau a phridd, wedi’i bennu ar y gyfradd is oherwydd ystyrir ei fod yn annhebygol o achosi llygredd tir, dŵr neu aer.