1. glaciers flow down pre-existing V-shaped river valleys under gravity - the mass of the ice has far more erosive power than a river and so erodes the sides and floor of the valley
2. as it moves (gravitational potential energy) it plucks the rock from beneath and those rocks rub against (abrasion) the bed of valley, eroding it further making the shape become deeper, wider and straighter
its long profile is a realist of compressing flows making the valley over-deepened to form rock basins and rocks steps, particularly evident where there are alternating bands of rock of different resistances on the valley floor (the weaker rocks being eroded more rapidly to form the basins)