From the Expert Guide:"Napa Valley forbids the cultivation of any slope greater than 30 degrees, which also limits vineyard size. That said, a handful of even steeper sites that were planted before this ordinance have been grandfathered in and can still be spotted among Napa’s many hillsides."
Anyone know what these grandfathered steep vineyards are? Reading it got my curiosity up when I couldn't name any off the top of my head.
On the Sonoma side ( I believe the slope imitations are the same) there's Martinelli's Jackass Hill and the Hidden Ridge / Immortal Estate up in the Mayacamas