Might have to put in a brick wall then cover it up with the timber to make it look nice, the earth is always shifting.

Frost heaves the soil and causes shifting. The frost line here in Chicago is 36". To build a solid fence post here, one goes 42" down - makes a good solid and STRAIGHT fence line then.

You could do the same by driving rebar down into your soil to a dept below your frost line for NC. Drill holes in your timber and pin them on the rebar.

PS - coat the bottom timbers surface with an asphalt based foundation coating first, to prevent a sponge effect. Water logged timbers will freeze, then melt, then shift like soil, when the frost come out of that too :-)

What you want is called a retaining wall.