A normal fault is a fault where the hanging wall block moves down along the fault plane relative to the footwall block. Some normal faults have planar surfaces, and some have curved surfaces. Listric fault systems with steep upper parts and shallowly dipping lower portions are common in Mesozoic and Cenozoic basins. Normal faults are typically extensional structures. When two normal faults dip towards each other, a graben can form through subsidence between the faults. In addition to crustal movement, gravity is sometimes responsible for the formation of small normal faults.