Friction marks on surfaces of blocks (1 to 10 m) in pyroclastic flow deposits generated by lava dome collapse in the 1995–1999 eruption of the Soufriere Hills volcano, Montserrat, consist of a thin (5 to 10 mm) layer of cataclasite with a polished slickensided surface. In some examples, thin surface layers and veins of pseudotachylyte are found. The friction marks occur in several orientations on all block surfaces. They are typically 0.5 to 10 cm wide and up to ∼1 m long. The marks are similar to tectonic structures on faults. They indicate that the blocks were tumbling and sliding within the flows. Large transient stresses result when the mass of a large sliding block is applied to a small contact area on a neighboring block or the ground. Compositional data indicate that the pseudotachylytes involved melting of the flow matrix. These observations support models of pyroclastic flows as rapid granular avalanches.