Extension of the Earth's crust is necessarily related to normal faulting. Causes of the necessary extension are obscure, however, and have been the subject of a variety of explanations. In northeastern California there is strong indication that the extension is the product of deep-seated right-lateral shear deformation related to the Walker Lane. Tectonic evidence, sharply etched in volcanic materials, suggests that (1) in a broad zone of lateral shear, normal faulting is due to the dynamic tension of the shear deformation and will differ in nature from that produced by compressive stress fields, and (2) an older set of Walker Lane lineaments exists which has been rotated away from present lateral stress and is now under constructional controls. Northeastern California provides evidence for the hypothesis suggested by Wise in 1963.