Glaciers formed in the canyons and, as they advanced and retreated the canyons were deepened and carved into U-shapes.
(Glaciers; formed; L:in the canyons; T:as they advanced and retreated the canyons were deepened and carved into U-shapes) (they; advanced; ) (they; were deepened; ) (they; carved; into U-shapes)
Glaciers they they they
formed advanced were deepened carved
L:in the canyons T:as they advanced and retreated the canyons were deepened and carved into U-shapes
into U-shapes