  Glacier and ice-sheet motion is fundamental to glaciology. However, there is
still no clear consensus for the optimal way to describe glacier and ice-sheet
sliding. Typically, sliding is parameterised using a traction coefficient
nominally linked to a given theory describing one or a limited set of sliding
processes. However, this approach precludes the possibility of multiple
simultaneous and spatio-temporally varying sliding modes with inaccuracies
resulting in model error propagation as the system evolves away from the
conditions under which it was optimised for. Here, revisiting early theoretical
work, we describe glacier sliding as a scale- and setting-dependent 'inner
flow' that arises from multiple overlapping sub-processes, bridging divides
between hard and soft beds, rough and smooth beds, and stick-slip and
continuous sliding as well as providing a consistent definition for form drag.
The corresponding 'outer flow' then accounts for 'normal' ice deformation. We
propose that the significance of 'Iken's bound' is then reduced if form drag
dominates over subglacial cavitation in a given region, possibly explaining the
persistent functionality of Weertman-type sliding in process-agnostic sliding
studies over rough topography. Last, reviewing observation-based studies we
suggest that a simple 'unified' sliding relationship controlled by a single
tunable coefficient may not be a realistic prospect, but that a Weertman-type
relationship with careful consideration of the power value is presently the
'least bad' option.
