The rocks observed on the Yalours and surrounding areas are predominantly igneous intrusives that have cooled deep under ground. The main rock type is grey to black gabbro composed of small crystals of grey-white feldspar and black pyroxene. The rocks here were formed by much the same process that the rest of the peninsula was formed; the Pacific plate sliding under the Antarctic Peninsula in an ocean/continent subduction-type tectonic event. This process, called orogenesis, is one of the most common ways for a mountain chain to form. The Andres and the coast ranges of North America are other classic examples of this type of tectonism. The mountain building has nearly stopped though; it is only at the very tip of the peninsula and in the South Shetland Islands that the process continues. For the rest of the peninsula, it erosion and gravity’s turn to shape the landscape.