I'd imagine the sand was used to set the rocks... Rather than being used to level anything.
If you want to see what they used to level the soil with, you'd have to lift some of that sod.
When you say that you want to plant along the wall... Do you mean that narrow strip between the turf and the rocks? or... were you planing to make a decent sized bed by taking up a bunch of the turf?
by how green the grass looks, I'd say that you would stand a good chance of growing anything you wanted once you cleared some ground.
I can't imagine digging a trench and removing soil and then bringing other soil in...
I've never had to do more than take up the grass and shovel turn the soil beneath... Ideally, I find a source of manure / compost to turn in with the existing soil... And... plant, mulch, enjoy.
