Paso Robles is, in my opinion the northernmost outpost on the Central Coast appellation. I know that technically, it goes further north, but here is my reasoning: The Central coast is defined by the 3 transverse river valleys, Santa Ynez, Santa Maria and Edna Valley (about 30 miles south of Paso). It's these valleys, and the funneling of the marine influence east, that provide for the wildly diverse microclimates. The reason I include Paso is because of the Templeton Gap (excellent commentary on the topic from Jason Haas of Tablas Creek). As the mountains move from east/west to north/south, the Ocean breeze swarm north along the Santa Lucia Range. North of this, the climate is different, and a little more uniform based on elevation and proximity to the Ocean. I'm sure some educated veteran will jump all over this and give me 10 reasons why my classification is wrong. That's fine, but it's my take on it, and for my me to wrap my brain around this area, this is how I call it.
A lot of hay is also made about the Eastside vs. Westside debate. By many people's opinion, the 101 freeway bisects the region, and really makes it 2 separate appellations. It's like "Zin to the right, Rhone to the left". This is oversimplified. In reality, it's a serious of triangles. Remember that Templeton Gap? Well, as it pushes cool air north it gets trapped in all of these nooks and crannies. In theory, you could have a warmer microclimate West at a higher elevation than a high elevation further East, because the winds never make it to the higher elevation vineyards in the West but dissipate as they travel East with a constant low breeze. whew.
And then there's the soil. We all know Limestone is a buzz word, and I had always heard about the Limestone was a major reason for Tablas Creek and Calera further North selecting their vineyards. But I was always a little wary of how much Limestone could really be here. Ok,it's actually Calcareous clay (thanks again Jason), but for our intents and purposes, it's limestone. Well, there's a lot. As the plates collided to form the Santa Lucia Mountains, the former seabed on the south got shoved into the northern plate to give a ton of seafloor fossils and limestone, particularly in a crescent shaped formation stretching from Lompoc about 75 miles north, primarily on south facing slopes. Ask a vineyard manager about how often they find whale bones. What's great about limestone? Well, without getting too technical (and I've read up on this) there are 2 basic reasons why limestone is great a) it is a unique soil type that retains moisture, but only gives it to the plant in times of drought, and is a well draining soil that keeps it away from the plant during time of heavy precipitation. It's like internal drip irrigation without the soil erosion. b) limestone appears to transfer more vital nutrients directly to the grapevines more efficiently. This ultimately seems to help natural acidity develop better in the wines. For me personally, I observe a savory, umami quality in wines that grown in limestone, but then again, I may just be projecting.
There is still plenty of bad wine in Paso, and even more, solid, commercial quality inexpensive grapes grown in the Northeast of Paso. But the real story is the diversity of grapes and microclimates available here. It's so diverse that there isn't one variety or group of varieties that has emerged as the showcase. Rhone, Bordeaux, Italian, Spanish, Zin, Petite and even Burgundian varieties are all excellent from top producers. Contrary to my previous skepticism, Paso is a world class grape growing region that is still in it's world class infancy.