My experience predates chemical herbicide usage, so that might work now,
but without it, both the blackberry and maple shoots will keep coming
back unless you remove all the roots and stumps; that means digging and
probably repeating the process for a couple of years.

I don't see gravel being a problem; plenty of gardens and yards contain
some gravel, and as long as it doesn't interfere with cultivation, I
would just let it go. It can actually help drainage. If you really
want to remove the gravel, build a sieve out of appropriately sized
hardware cloth, and run the dirt through it while you are dealing with
the roots.

I spent years getting rid of a blackberry patch; the berries were nice,
but with young children I didn't want a patch where they would get all
scratched up. I chopped down a small maple that shaded where I was
putting my garden, and 35 years later it still sends up shoots, which I
cut and use for garden stakes.

tenplay wrote:

In my backyard, I have an area roughly 20X30 that is covered by some
blackberry vines, maple tree shoots, and gravel mixed in with the dirt.
What is best way to reclaim this area so that I can use it for a garden
or lawn? I have no idea how it got to be this way before we moved onto
the property. Thanks for any advice.