Pixguy,
We are on ledge here and here is a picture of the barn. and some rocks..
You need to have a clear level area 30x40' and remove the base material then build up a footing / slab and compact it.
The ledge you and I have is going no where so build on it, but make sure you have a decent base.
What you dont see in the pictures is the ledge 10' high behind the barn, but we have plenty of rock and ledge here and I just work around it and am happy when I can dig a few feet and not hit it..
So scrape down your area to a flat spot then bring in some crusher run gravel and pack it - you will be fine with a slab on grade barn.
Carl