You can use segmental retaining wall block(SRWs) that do not have to be below frost and should never be placed on a concrete footing. They have shear lugs cast in the block for stability and the tops can be stepped down as needed. - Of course, the first course should be placed on a compacted gravel base.
There are numerous different styles available and best and most common are Allan Block, Anchor Wall Sytems, Keystone and Versalok and all have good web sites with design and construction details and methods. This type of block, if engineered can be built up to 40' high without a concrete footing, but using geo-grid fabric for the taller walls.
Last edited by mudmixer; 01-25-2012 at 08:01 AM.