Vertical slice through a scaled physical model of a salt diapir. The model conists of alternating sand layers of different color placed over a viscous layer of putty, which is used to simulate salt or ductile shale. The lower sand layers were laid down before the putty started flowing upward; the upper layers were added during flow. Slide courtesy of Bruno Vendeville of the Applied Geodynamics Lab, Bureau of Economic Geology, University of Texas at Austin