Wood Retaining Wall Plans

I intend to build a wooden retaining wall with its 20 feet long .... My question is the depth of what I have to dig below grade (ground?) What I put in my first row of wood, IM planning on using ... . Thank you for 6X6!

I would start four feet deep, especially if you live where it is cold in winter. 6x6 would be great, but be sure to use treated wood. success

Gravel in the woods. Depending on where you live and the depth of frost heave will depend on the importance you have to dig. For wood they recommend boulders over gravel just below the surface. Do not forget the T (called Deadmen) you will need to put all four feet to anchor the wall against the dirt.

I would put a few feet below the growth. you could put the rock bottom. Also make sure that the water from the wall of a place to drain.