I used railroad ties. Keep in mind what ever method you decide upon that the water will migrate into the soil behind the wall there fore you need and extra 5 to 6 foot of distance from the tree to account for this if you're trying to keep the roots dry. Here is a link of our wall after it was built.
http://www.rockytoppers.com/the_pond.htm


The road goes on forever and the party nevers end...............................................