Sentence:  [View Larger Version of this Image (130K GIF file)] Discussion The martian landforms resemble terrestrial gullies, which form by a combination of processes, including overland flow, <WSD>headward</WSD> sapping (32 ), debris flow, and other mass movements (33 )--all processes that, on Earth, involve the action of water. 
Meaning of the word marked with <WSD> tags:  Cutting backwards or upstream above the original source.