Searching for more information on Flat Rock based on Vic Loudonís sketch drew a blank until, by luck, I received a contribution for Poetís Corner from Melisande Luna of California, who also sent me the following: 

Flat Rock winery is near Flat Rock beach in northern Mendicino County, on the extreme north coast of California. I believe this is the Cascade volcanic geoprovince. Being due west of (and at lower elevation than) Mount Lassen, the soils should comprise chemically and weathered volcanic alluvium. (mostly felsic andacites, but with some local basalt flows and ash lenses).

[It must be these volcanic soils that give it its positive flavour full of sweet blackcurrants... and I thought they used grapes! -Ed.]