Sheets of deformed beds in the Eocene flysch of the southwestern Pyrenees are exposed along strike for over 16 km. The thickness of these sheets in places exceeds 100 m, and their volume is c. 10 km3. The sheets formed by gravity induced slumping, triggered most likely by earthquakes. The velocities of slump transport may have been as high as those of turbidity flow. Directions of slumping are consistent indicators of regional palaeoslopes. The slump sheets do not reflect a particular environment of deposition, but more likely record the seismic regime (periodic earthquakes of great magnitude) of the flysch basin.