Siltstone. The rock exposed in this cutting is a siltstone of the Orange Grove Quartzite formation, Witwatersrand Supergroup, formed by the accumulation of under water of particles finer than the usual sand. It is red in colour due to the presence of iron oxides, and consists of layers of slightly differing hue, resulting from the deposition of differing proportions of minerals. These layers were close to horizontal during the orignal sedimentation, but have since been tilted and now dip southward.