You'll have to define "hardpan"..

The area where I want to plant trees has strange soil. The top layer
of about maybe 6" consists mostly of the root zone of the grass.
Immediately below that is about 8" of almost impenetrable hardpan. I
can't dig through it - I have to use a sharp-pointed hand tool to pick
my way through it. Below the hardpan is moist (not wet) clay which
rapidly transitions to mostly coarse sand and rocks. I'm not sure I
understand the proper way to "amend" such soil or how to create a
transition zone.

I can't imagine how anything could grow there but it does. Five years
ago there was nothing there but thin grass and weeds. Then I stopped
mowing it, and now there are sassafras, black cherry, black locust, and
maple trees growing there, all wild. One of the black cherries is
over 6' tall. A couple of the sassafras are over 7' tall. One of the
black locusts is over 12' tall. The maples appeared later and most are
less than a foot, but there is one that is about 5' tall, and thriving.

Two years ago I bought a 5' "autumn blaze" maple at the local
mega-mart and planted it in this area. Not having the least idea what
I was doing, I dug a deep (not wide) hole and backfilled it with dead
leaves and other organic matter gathered from the woods, and stomped it
down. I set the tree on top of that and threw in a bag of "garden
soil". When I was done, the tree's root flare was about 8" below the
surrounding surface, so I filled the rest of the hole with crushed
limestone to make it level. This little maple has been growing like a
weed ever since. This spring it's going nuts, putting out new growth
everywhere at an amazing pace. Every couple of days it seems there's
another set of leaves opening up at the tips of the branches. Now
that I'm learning a bit more about what trees need and how they should
be planted, I'm wondering if this little guy is in for a rude awakening
when he discovers he is in a tiny little oasis in the middle of the
sahara dessert. Where will his roots grow? Under the hardpan? Is
this viable? Should I dig him up this fall and re-plant using a more
proper method? And, how do the other trees (the wild ones mentioned
previously) deal with this?

I'm at the point in my life where I find all this gardening stuff
fascinating. There's so much to learn. I've never really paid much
attention to it before.

I had to laugh when you mentioned renting a jackhammer. That very same
thought had just occurred to me last week. About three years ago I
had a guy over here with a Kubota with a backhoe to dig a 10-foot
trench to bury some drainage pipe. He had a tough time breaking
through the hardpan. He had to strap the back end of the machine to a
hickory tree to get enough traction to get the bucket to break through.

In your earlier post you mentioned "sun splits" and that reminded me of
something. I think that happened about 2 years ago to the maple I am
considering moving. There was a vertical split, about 7 inches long,
along the main trunk, about 2 feet from the ground. I can't remember
what time of year I first noticed it. Being concerned about insect
infestation, and being largely ignorant about such matters, I sprayed
the split with an insecticide containing permethrin. The following
year, the two lower branches on that side of the tree died.
Coincidence? Or direct result of my spraying? At any rate, the split
has now healed over, and the tree appears quite vigorous this year,
with a lush set of leaves. Inspired by your earlier post, I went out
this afternoon and did a little pruning. Just removed some small limbs
that were growing at the wrong angle and threatening to choke or get
choked by other more desirable limbs. Still too timid to be doing any
major surgery. I'll have to work up to that.