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strain marker A natural object whose original geometry is known and from which the magnitude and orientation of the principal strain axes may be determined after deformation. Many objects have been used, e.g. fossils (crinoids, belemnites, corals, ammonites, brachiopods, and trilobites), rock textures (i.e. ooids), conglomerate pebbles and clasts, single crystals (e.g. feldspar megacrysts), spots in contact aureoles, volcanic lapilli, spherulites, and xenoliths.