  The observed higher uplift rates before the end of deglaciation requires the
existence of a low viscosity channel or layer. The uplifts observed after the
end of deglaciation does not show any contribution from this low viscosity
channel and a homogeneous viscosity model fits very well to the observed
uplift. Most of the researchers therefore prefer the homogeneous model and
suggest that the higher uplift rate before the end of deglaciation is the
result of elastic contamination. It has been shown that the elastic deformation
of the lithosphere is far too small to be responsible for the observed extra
uplift; therefore, the homogeneous viscosity model should be discredited.
  The homogeneous viscosity of the postglacial period and the high uplift rate
of the late glacial period can be explained with a model which has an upper
layer determining the homogeneous viscosity and the layer below it which has a
low viscosity. The contribution to the uplift of this low viscosity layer is
indistinguishable from an instantaneous uplift.
  Current GPS deformation measurements indicating 10^21 Pas average viscosity
for the lithosphere, which is consistent with this model. The viscosity of the
low velocity zone should be between 10^18 Pas and 10^11 Pas. The lower
constrain on the viscosity of the low velocity zone can be deduced from seismic
observations. This viscosity model is consistent with post glacial rebounds and
further supported by the lack of the dynamic topography.
