>> Can you put a steel post into the ground there?
In my yard, it depends on where I dig and how deep. I was able to pound in some Tee posts, at least 6+ inches deep (maybe 8", I'm not sure). They seem really solid - I think you could make bricks from this clay.
I suspect my yard was treated like Natalie said: they bulldozed the whole area, and some spots got more rock than other areas. I was able to dig one area mostly with a shovel after the clay had been loosened - say, 10-20% rocks. I went down 1.5 to 2 feet there!
In another spot, just to get down 8-10 inches took many passes with pick/mattock/rake, pick/mattock/rake, pick/mattock/rake. Let's see, that was somewhere around 60% rock, 25% juniper roots, 15% clay.
That was one place where I decided that the beds could mostly raised ABOVE grade for a few years. Unfortunately, the existing juniper/rock formation was already raised somewhat above grade, and I didn't want a bed-on-a-pedestal .
You can't pound on rocks with a mattock blade, you'll dent or chip it. You can't pry juniper roots no matter how much leverage you have: you'll pull an arm out of a socket before the roots come out of the ground.
Pry a few rocks, chop a few roots. Repeat until you get enough clay free to be worth scraping it away.
Repeat.
