...and in fact glaciers flow more swiftly near the top, as can be determined by hammering a flexible metal rod into a glacier and leaving it for a year or so: the rod will be bent at the top in the direction of flow.
The glacial horizontal velocity is greatest at the top because the that is the sum of all the flowage from bottom to top, not because the the plastic flow at the top is the greatest. Plastic flow will approach zero as depth approaches zero.
The below left is, as I understand it, your image of the bent rod (flow direction left to right); The below right is how I believe it would actually bend:
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Added by edit: The above right is also wrong - See message 72 for what I think is correct.
First of all, I blotched the "bent rod" diagram. It should have been:
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The horizontal displacement at the bottom is greater because the internal plastic flow is greater.
The other diagram in message 69...
...illustrates the total displacement in a certain amount of time (the vertical velocity profile). As it says at diagram bottom: AC is total movement, AB is sliding at the glacier base, and BC is the internal plastic flow. Actually, the various BC's are summations of all the internal plastic flow from the base up to a given height. The vertical velocity profile line approaches vertical at the ice top surface because the plastic flow rate approaches zero.
The ice at the top may have the lowest rate of flow, but doesn't it also have the highest velocity?
The top ice has the highest flow rate (velocity) relative to the base of the glacier (or in general, the fixed rock). That's because it is the sum of all the flow rates, bottom to top.
You can't really see the vertical flow rate profile because it's buried in ice. You can see the lateral flow rate profile. But the essence of it all is that glaciers advance by some combination of basal slip and internal plastic flow. I believe that in some cases the glacier base can frozen to the rock below - There is no basal slip component. If there is no basal slip component, then there is also no erosion happening.
By the way, glaciers retreat when the front ablation rate (melting etc.) exceeds the flow rate. The glacier does not start flowing the opposite direction.
Added by edit: I'm still not sure I didn't blotch up some of that plastic flow theory.