At the same time, the glacier grinds away the valley walls and straightens sharp bends, changing the V-shape of the valley to a broader U-shape.
(the glacier; grinds; away; the valley walls; T:At the same time) (the glacier; straightens; sharp bends; T:At the same time)
the glacier the glacier
grinds straightens
away the valley walls T:At the same time
sharp bends T:At the same time