Terry Coombs wrote, on Fri, 23 Jan 2015 09:33:40 -0600:

There are ways to have a seamless curved transition between
the deck and the backsplash

That's why I'm trying to backfill.

BTW, it seems to be about 1/2 inch, maybe slightly less.
I can get my index finger in there, but not my thumb.

I just want to get rid of it, somehow.
The best I can think of is caulk, but I do agree that I've seen
mold UNDER caulk, so, it might seep under anyway.