Mass Movement – The downhill movement of weathered material under the force of gravity. The speed can vary considerably
The mass was moving (mass movement) in more ways than one – everyone was falling downhill!
The speed of mass movement can vary considerably depending on the particular type of movement:
Soil creep – where movement is barely noticeable
Slumping – rapid movement
Sliding (or mudflow) – rapid movement.
Rotational slumps are a common type of mass movement on coastlines. Wave action erodes the base of the cliff, removing the cliff's support. Rainfall infiltrates the slope above the cliff through unconsolidated porous material and creates a "slip plane" where it reaches an impermeable material, such as clay.
The increasingly heavy, saturated material above the clay slumps down in a mass movement.