Snakes, fungi, mould and guano...

If you don't mind departing just a little from the Tome to get this one thing the way you want it to be, you can add a Color Key sheet effect to the CONTOURS (SEA) sheet, move it above the Edge Fade Inner sheet effect, and then draw a solid polygon of magenta where you want the hole in the contour to be.

Make sure the Activate Sheet Effects box is checked and ok.

@Bidmaron - This is the spot where I started looking, and the search for all the possible textures and symbols took me 8 weeks. There's no guarantee that I got them all, either.

https://maps.arcanum.com/en/map/belgium-1777/?layers=37&bbox=527930.4200425778%2C6537602.115436598%2C537499.3805210282%2C6541285.424737089

I might be getting slightly off topic here, but I made some rocks that can double as cliff material.

But we do need the stuff to generate the right sort of mood, so maybe not?

I had to stop going there. Too many hours spent gazing already.