According to HerHusband :
That sounds like good wood, but here in California I don't think it's
available. 4" tall strips and that thin would be hard to make work in
a 1' tall wall though? We have redwood of course but quite expensive
these days, and the 2" thickness that I'd probably need to use might
not curve along the hillside contours very well.

I wonder if those synthetic deck boards would work? They're more flexible
than wood, and aren't supposed to rot.

I'd expect them to slowly bow out further as long as the dirt
was pushing on them.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.