There must've been a heavy rain. Washed it away.
5 seconds sound clip from the The Cowboys (1972) movie soundboard.
You can hear this line at 02:11:18 in the Blu-ray version of the movie.
Quote context
[...]
- Well, it's usual to say something like, Rest in Peace. Eternal Sleep. In Fond Memory of...
- Was he a family man? Did he have any children?
- Yes.
- Well, then I carve: Beloved Husband and Father.
- I don't see it. It's gotta be around here somewhere. I remember that tree.
- There must've been a heavy rain. Washed it away.
- We ain't never gonna find it now.
- Well, come on, we're burnin' daylight.
[...]