Window frames

[Small]

How do I fill a gap between the bottom of a wooden sash window frame and a concrete (exterior) window ledge. I was going to use Wood filler but the gap goes back a fair way and there is moss and loose debris under there.
If I use wood filler - I would need a hell of a lot of it to fill the gap and does wood filler bond wood to the concrete window ledge effectively? Any help appreciated. Cheers.