When the mountain cooled, glaciers periodically masked its flanks, carving out the U-shaped valleys there today.
(the mountain; cooled; ) (glaciers; masked; its flanks; T:When the mountain cooled; T:periodically) Context(glaciers masked,List([26, 54))):(glaciers; masked its flanks carving out; the U-shaped valleys; L:there; T:today)
glaciers masked,List([26, 54))
the mountain glaciers glaciers
cooled masked masked its flanks carving out
its flanks T:When the mountain cooled T:periodically
the U-shaped valleys L:there T:today