A <3 mm thick, intensely brecciated layer associated with Marinoan (c. 635 Ma) glacial striations on a Neoproterozoic sandstone pavement is documented. Only instantaneous solidification could have preserved the breccia in situ. Glacier speeds of c. 0.3 m s−1 would have induced peak flash temperatures sufficient (>1130 °C) to cause partial frictional welding of the deformed quartzitic bedrock beneath an ice-loaded clast. These results support suggestions that glacial seismicity is due to episodic very rapid shifts of large ice masses over the substrate (stick–slip movement). They also indicate that significant effective normal stresses occur at the base of glaciers.