Bob, sandstone was used in most of the old buildings in town. I've only been in Graaff-Reinet for 23 years, and am not an expert in geology, but you might be right. The rock columns in the Valley of desolation are apparently dolerite, and as I understand, the Karoo region was under water during the Ice age, and millions of years ago, there was volcanic activity. There are lots of fossils in the area, which seems to support this theory. I know there were a lot of quartz crystals, some quite large, on my in-laws farm, and apparently that also indicate volcanic activity in the past.
It is funny that I live in such a historic place where we get a lot of overseas visitors, but I don't get enough time myself to explore it more, but who knows, perhaps in a few years’ time