Solifluction

Saturated soil flows downhill due to the effects of gravity.  In summer, the active layer of permafrost thaws, releasing water which saturates the soil and causes it to move downhill.  The greater the gradient, the longer the solifluction lobe which terminates at the base of a hill as the gradient is too low to support the slump.  The solifluction lobe becomes a solifluction terrace, creating the appearance of a natural staircase.

The geological term for solifluction deposits is 'head'.  Head infills valleys in periglacial areas and can be seen on BGS maps.