Geologically, the South Mountains are thought to be a metamorphic core complex: evidence of movement of the North American tectonic plates from southwest to northeast and northeast to southwest, pushing up a series of mountain ranges including South Mountain. Other ridges with parallel orientation lie within the basin covered by basin fill sediments. The structural basin forms the current Phoenix metro area, which appears flat like a lake around mountains that rise over it like islands. The buried ridges are in the same orientation as the South Mountains, about one km high, and about one km apart from peak to peak, perhaps about 15 of them underneath the basin fill.