The process of water eroding the ground to create gullies, or the formation of channels and ditches through soil erosion.
From 'gully' (a water-worn channel) plus the gerund suffix '-ing', which turns verbs into noun forms describing the ongoing action or process.
Gullying is a slow-motion disaster that farmers dread—each rainstorm deepens the channels a little more, and what starts as a small trickle eventually becomes an impassable ravine that ruins cropland.
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