When pressurized water is fed over such an intersection, the pressure pushes down the thin layer of rubber, separating it from the fluid below, and it clamps shut the fluid channel below, like stepping on a hose.
The question is one of the most hotly contested in palaeontology and revolves around an apparent gap in the fossil record immediately prior to the K-T boundary - the distinct layer of geological sediments separating the Cretaceous and Tertiary periods, and associated with a massive asteroid impact and global extinction event.
Instead of separating the wheat from the chaff according to what layer it is found in, this relies on the different optical properties of the various inks, the mould and the parchment itself.