The internal structure of intrusive and extrusive salt bodies elucidates their external shape and the mechanism and history of diapiric emplacement--information relevant to petroleum exploration. Trace amounts of brine can act like heat to weaken salt and promote geologic creep by solution-transfer mechanisms. We compare natural strain markers in salt with the artificial markers used by fluid dynamicists: stream lines are the smooth flow paths followed by fluid particles; stream surfaces separate flow laminae and contain the stream lines. Grain-shape fabrics in salt anneal readily and have strain memories so short that lineations map the stream lines and foliations map the stream surfaces of the last stages of deformation. By contrast, larger structures, like folded beddi g, have much longer strain memories. As a result, multiple generations of tonguelike, recumbent flow folds formed by unsteady flow in the mother source layer rotate to upright sheath folds or spines in the stems of diapirs. Here they are constricted and refolded into steep curtain folds to form crescentic and other interference patterns visible in the roofs of salt mines. Mature diapirs can form crestal bulbs, which may be balloon or mushroom shaped and which potentially form larger petroleum traps than previously supposed. Screens of country rock may be entrained between the stem and peripheral overhanging lobes caused by episodic sedimentation.