from what I gather, the bottom layer is old wood. I would use wood chips because they would be more dense.Excellent!
What did you put in the beds? Too deep for all "raised bed" bagged mix, that would likely break the bank.
what I have read, you can use old lumber, again, I would try to get green branches with bark on them.
or sawdust/shavings from a sawmill.
the top thin layer could then be compost and garden/potting soil.
after a time all that wood will turn into black soil.
that's my story and I'm sticking to it.. lolol