What is "wordface"?
Solution 1:
This is a neologism based on the term rock face.
A bare vertical surface of natural rock.
But because this surface is composed of tightly packed letters written on paper, Pratchett chooses to call it a wordface.
Solution 2:
Pterry is coining a neologism, making up the term "wordface" by analogy to "cliff-face." (Note the "rock strata" analogy.) He's just playing with the language.
(Later: Ah, missed it by that much!)