Rock block slides as large as 20,000,000 m³ occur in northeastern Pennsylvania where dip-slopes are undercut by rivers or by man. Slippage occurs along bedding in mudstone units where bedding dips out of the slope. The planar bedrock slabs are bounded by joints or the ground surface. The slab’s rectangular, arcuate, or triangular plan-view shape is controlled by joint and outcrop orientation on the slope. A 10⁴ variation in slide-block volume is controlled primarily by differences in slope length and block surface area. Some blocks slide off the slope and onto the flood plain, while others only open up fissures and remain on the slope. Blocks move straight downslope or pivot toward an unbounded or more undercut side. The slides are part of an on-going process dating from post-late Wisconsinan glaciation (18,000 yr B.P.) to present. The region is seismically inactive; three historic slides are associated with high moisture conditions, so prehistoric slides were also probably triggered by high cleft-water pressure.