Large boulder of gneiss and schist (high-grade metamorphic rock) cover alluvial fans along Highway 178 along the western flank of the Black Mountains. This boulder is a migmatite, a highly deformed metamorphic rock that has light-colored granitic intrusions (called aplite). Rocks like these form deep in the Earth's crust. Their exposure in Death Valley is a testament of the intense tectonic forces shaping the region.