For context: My characters are in a tunnel that goes through a cliff, conecting the jungle to the sea. They can no longer leave the way they came in, and are standing at the second cave-mouth, which at this point is a few feet above the ocean's surface. Then one of the characters notices the high-tide mark on the rock wall, and figures out that in a few hours, the whole cave will be filled with sea-water, to a level above both their heads.
The question I can't figure out is: will the rock wall be darker underneath the high-tide mark, or lighter?
Oh, for a bonus q: what would be the difference in distance (ie feet) between low and high tide against a cliff-face? Not sure it'll come up, but it would be nice to know, just in case.
I've checked wikipedia and fact monster, but all I get is defintions of the term HTM. I even tried googling for pictures, but all I find are pictures of beaches or shorelines - no tunnels or rock walls.
I suppose I can fudge the line if I have to, but I like this particular dialogue exchange, and I can't shake the dread of looking really silly over something so small.
Thanks in advance!