Much of the extra local stretching in Death Valley that is responsible for its lower depth and wider valley floor is caused by "left" lateral strike-slip movement along the Garlock Fault south of the park (the Garlock Fault separates the Sierra Nevada range from the Mojave Desert). This particular fault is pulling the Panamint Range westward, causing the Death Valley graben to slip downward along the Furnace Creek Fault system at the foot of the Black Mountains. The rocks that would become the Panamint Range may have been stacked on top of the rocks that would become the Black Mountains and the Cottonwood Mountains. Under this interpretation, as the Black Mountains began to rise, the Panamint/Cottonwood Mountains slid westward off of them along low-angle normal faults, and starting around 6 Ma, the Cottonwood Mountains slid northwest off the top of the Panamint Range. There is also some evidence that the Grapevine Mountains may have slid off the Funeral Mountains. Another interpretation of the evidence is that the Black and Panamint Mountains were once side by side and were pulled apart along normal faults. These normal faults, in this view, are steep near the surface but become low angle at depth; the mountain blocks rotated as they slid to create the tilted mountains seen today.