Sentence:  The earliest air photograph showing <WSD>cropmarks</WSD> at Mucking appears to be a high  level vertical view of the area taken by a German aircraft of the Luftwaffe in 1943.  This photograph, however, only shows a few small and indistinct marks. 
Meaning of the word marked with <WSD> tags:  A sign of the former location of some feature (such as a wall or a ditch) revealed by variations in plant growth on the surface.