ABANDONED. YA fantasy, Siblings walking through mist/fog

[Naomi]

I was 13 at the time we read this in School (1999) in the UK. All I remember was that it had a brother and sister who were teens. They were walking through a fog or mist. On their journey the boy dropped a stone to see how long it would take to reach the ground. Later on they came to three pillars or towers. We got to the point where something was on top of one of the pillars. I can't remember the rest.

[Lobstergirl, au gratin]

Naomi, are you still looking for this or did you find it?

[Kris]

No response, moving to Abandoned folder.

Naomi (OP) was last active on the site in November 2019.