Have you looked at the Alternative Energy Centre near Macchynlleth?
They're not as high as you, but they did create a veg garden out of,
basically, slate. *I saw it in the early days (1970s) when I remember
some unhappy looking soya beans in a bed of slate chippings, and again
15 years later when the veg were flourishing. They might have some
advice for you.

--
kay

Almost right Kay, its actualy The Centre for Alternative Technology,
Llwyngwern Quarry, Pantperthog, Machynlleth, SY20 9AZ
,
http://www.cat.org.uk/
I've visited them several times over the years and have seen the place
change a lot over around 25 years,
Well worth a visit if you are in the area.
David