Within the Silurian rocks of southeast Mageröy, Norway, there is an unusual psephitic fades containing pebbles of sandstone, limestone, and metamorphic rocks. During late Caledonian orogenic deformation the rocks were folded and cleaved, and in the conglomerate the contrasting ductilities of the constituent pebbles resulted in differing patterns of strain. In the first stage of the deformation, the sedimentary pebbles deformed principally by extension parallel to b while the metamorphic pebbles underwent discrete physical rotation into the lineation direction. In the subsequent stage of irrotational strain, the sedimentary fragments were flattened in the layering while the metamorphic ones yielded by brittle fracturing. The behavior of the metamorphic pebbles provides a key to the deformation history, enabling the other tectonic structures like folds, cleavage, joints, shear zones, boudinage, and deformed fossils, to be assigned to positions in the movement plan. The role of the rigid metamorphic pebbles in the deforming conglomerate is analogous to photoelastic stress meters, and from them one can predict the orientation of the principal stresses in the final flattening phase of deformation. The consistent pattern of fracturing of the rigid pebbles suggests a uniformity of strain throughout the domain studied.