First from Stephen King's On Writing, where he likens the act of writing to unearthing a fossil:
Dig for the bones and see what they look like.
(paperback edition p.200)
Then there's Anne Lamott in Bird by Bird
... I am the designated typist, and I'm also the person whose job it is to hold the lantern while the kid does the digging. What is the kid digging for? The stuff. Details and clues and images, invention and fresh ideas, an intuitive understanding of people. I tell you, the holder of the lantern doesn't even know what the kid is digging for half the time -- but she knows gold when she sees it.
(Anchor Books paperback p.56)
Does anyone know of any other references to writerly digging?