Other poster likely has a point about hill country temps. Ours was already long dead (see below) by the time of the Houston area's two ~25° freezes of 2017. Otherwise my answer was --> In Houston I believe we killed the beautiful young avacado plant our neighbor gave us by planting in an almost flat bed where it was actually in a flat or slightly depressed spot and therefore sat in too much water. So, to me it sounds like your soil over rocks (Really solid? I was picturing a, by shovel or whatever, impenetrable layer of small to medium sized rocks) is fine, as I imagine it drains well. We get lots of rain so make sure it gets required water too though. Not a pro, just my two cents / theory.
I have one in NE area, still in a pot. Tried to grow several over the years on the ground but lost them to the occasional temperatures in the 20's. Unless you can bring them inside or have the means of providing some heat during the freezes it may be very difficult to near impossible to keep it alive. But, I'd just go to the nursery on a Saturday when they are broadcasting the radio show and personally post that question to the experts at hand. Best of luck to you!
I planted an avocado in a raised bed in the North central area of San Antonio. Clay soil rather than rock. Your main concern will be the weather. Some Avocadoes are bred to survive some colder weather. Hill Country freezes and lower winter temps may be an issue.
You will have to call and talk to professional.
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