[TowerTalk] Guy anchors in shallow soil

[Linux Mercedes]

So I have been trying to put up a 50' 25G tower for a little while now. 
I bought some 6'x8" screw anchors thinking they'd go into the clay soil 
pretty easily, and they do...for about two feet. Then they hit rock; I 
am pretty sure it's bedrock or something similar from talking to my 
neighbors. Obviously that's hardly enough earth for a screw anchor to 
hold on to.

What are my options here? Looking at Rohn's drawings, I am entirely sure 
that going down 4' for a concrete base is out of the question. I am 
currently thinking I'll bust a hole about 8" around down maybe 3', dig 
an angled trench for the bar of the screw anchor, and set it in the hole 
with a bag or two of concrete. Hand digging that hole doesn't sound fun, 
though.

For what it's worth, I'm in central Missouri.

Thanks!
--Nathan