Title: A Traveller's Riddle
Fandom: Silmarillion, Hobbit, Exeter Book
Challenge: Horizon
Other prompt: N/A
Rating: G
Length: 33
Content notes: N/A
Author notes: Thanks go to Zhie & Runa for encouragement & sanity-checking.
Summary: Who am I?

On Mountain-ridges, treetops, there I bide
On sea of grass or water, shoaling beach,
I stand between, the elements divide
A line that one can see, but never reach

Who am I?

[Answer in first comment)