Why exactly is a glacial valley u-shaped?

U-shaped glacial valley (taken on a recent trip to the Austrian Alps)

Glacial valleys as well as fjords have a U-shaped cross sectional profile.  For the most part this stems from the fact that energetically it is easiest for ice to flow down and get channelized in a pre-existing river valley.  Once in those valleys, ice is proficient at tearing apart the landscape underneath and moving large portions of the landmass. In these pre-existing v-shaped river valleys, the glaciers act as widening, straightening, and overdeepening  (cutting down vertically) agents.  The steep walls in a U-shaped valley come from the fact that the glacier flowing downslope is grinding the sidewalls as it moves downhill.  U-Shaped valleys have wide bases because of the widening effect of a flowing glacier and its resistance to flow and shear stress (high viscosity).  Simplified, this means that ice, being a solid, (but still acting as a fluid, important distinction) does not get channelized as highly as the water did in the valley before.  It follows that the bases of U-shaped valleys become even more flat because of sediment infilling.  With sediment infilling, what would have been a more parabolic shape, gets its trough filled in by sediment because the glacier is doing so much grinding of the preexisting valley to deepen, widen and straighten it, that there is bound to be a large sediment flux.  Also, that 'overdeepening' is why you usually have glacial lakes sitting in a u-shaped valley after the glacier has receded, because there are low spots conducive to have meltwater and runoff sitting in them.