Termites are a likely problem? Non-treated wood? Not familiar with locust fence posts (Gleditsia or Robinia?). Maybe osage orange would work. Also, if you do use them, you can avoid the work of smoothing them to fit by using a double layer of geotextile fabric as a "soil separator" behind them (attach it as permanently as possilbe to the logs).

I am not familiar with locust, but anytime we have used non-treated wood, it has eventually needed replacing. Even yellow heart cedar (which was sold to us as a 25 yr lasting product), degraded after about ten years. So, if you foresee staying there for 10+ years, are you prepared to replace this locust border in time?

I've seen some really neat landscapes done with log/sapling sections placed upright, side-by-side, sometimes for pretty long raised beds. It looks fabulous. Obviously it's not a permanent material, but so what? Maybe you could erect a new wall of similar or alternate material around these once they rot, just burying them in place.

KarinL

I used some oak logs standing upright to make a grade change. After 10 years, it could be replaced. I'm happy, but I didn't pay for the logs. It was cheeper to use them than get rid of them. I did this in a woodsy section of my yard. If you are going for a long term, manicured look, I wouldn't recomend it.