Abstract
Since the classic work by Hubbert & Rubey it has become generally accepted that thin but extensive sheets, or blocks of rock may glide down a gentle slope only when the fluid pressure beneath the block is abnormally high. It is pointed out that the initiation of movement of the block can be explained if it is assumed that the fluid pressure, below the block, locally and for short periods exceeds the gravitational load. Furthermore, it is argued that when the glide block encounters resistance to forward motion, fluid pressures which also exceed the gravitational load, act within the glide block and are instrumental in the initiation and development of listric faults.