Mary Baker (MB): Dr. Rice, how is your book organized?

Thomas Rice (TR): An introduction describes the terroir as seen through the eyes of early ranching, farming and winegrowing families. Chapter One discusses environmental contrasts between the east side and west side, and general issues of water quality, wildlife habitat and native plants.

The remaining chapters are organized by region: Templeton Gap, Adelaida Hills, Salinas River Valley Terraces West, Estrella River Terraces, Salinas River Terraces Southeast, and the Creston/Shandon Area. Terraces West would include the San Marcos drainage area, and the Dusi Ranch vineyards. The Southeast Terraces are possibly two separate regions -- Santa Margarita and Huerhuero River Terraces. Although there are not many wineries in that region, there are big vineyards, with abundant geologic and soil variability. This region encompasses vineyards as diverse as Wild Horse, Maloy O’Neill, and Chateau Margene.

There were redwood forests here during the Pleistocene (Ice Age), and huge flood plains which incised the land and were later uplifted into river terraces. There’s been a lot of folding and faulting of the land, causing deeper sedimentary rock layers to be forced underneath the plains. So, for instance, in the high plateaus and orchards of the El Pomar region, the eroded tops of the highest points may be calcareous shale and mudstone, with surrounding recent alluvium and a veneer of Paso Robles Formation (Pleistocene Ice Ages) alluvium consisting of mixed sand, gravel, and clay covering the sedimentary rock layers.

MB: How long have you been working on this book? What was the basis of your research?

TR: I’ve been working on this book for over twelve years. Tracy has spent four years on it. She is handling the cultural history aspect of the book, and is doing a fantastic job interviewing local vintners and viticulturists. We hope it will be a book that anyone interested in the region or in winemaking will find interesting. I have been “digging holes” throughout Paso Robles for over twenty years, and the book includes research from my own projects and also students’ projects which I have supervised.

MB: Would you describe the broad geological differences between the central coast and northern California? You mentioned previously that the San Andreas Fault curves off into the Pacific west of Napa, resulting in more volcanic rock in Napa, whereas Paso Robles is almost entirely sea floor. Are there other differences between north and south California?

TR: There are many geological differences between the two areas. Napa is part of the North American (NA) plate, which is an expansion zone in Napa Valley, moving slowly northward. It features volcanic rock, gravel alluvium, and shale layers. The coastal splinter of western California from the San Francisco Bay to Baja is a compression zone. It is being forced against the NA plate and is moving slowly northward along the San Andreas Fault, grinding against the plate like cheese against a grater.

San Andreas is more than just an inconvenient fault. The surface terrain of western California is shaped by folding and faulting, but much of the shoreline is created as various layers of the Pacific Plate slide under the North American Plate in a process called subduction. As these tectonic plates are pushed under the continent, shavings of ancient ocean crust containing coral and marine layers curl upward, creating new California coastline, and simultaneously pushing against it. This process is referred to as the San Andreas system, and it’s responsible for the parallel cracks and faults in much of western coastal California.

The Pacific Plate is comprised of ancient seabeds and shallow coral beds over oceanic volcanic crust. After being pushed up against the NA plate, shallow seabeds form again and recent sea floor materials are deposited on top of older seabeds, resulting in the rich marine layers and fossils which we find in the Monterey Formation rocks located around Paso Robles.

The Central Coast region was once like a huge Puget Sound, with embayments of shallow ocean life living in large marine estuarine systems, later to be deformed by the grinding pressures from different directions (north, south, up, down), due to movement along the San Andreas Fault. These Monterey Formation rocks were populated with ancestral marine sealife (fishes, corals, plankton) and ocean mammals like seals and whales.

MB: How would you compare Paso to other wine geo-regions throughout the world?

TR: There are geologic formations (ancient marine embayments) similar to those in Paso Robles throughout France and Italy -- in Tuscany, the Rhone and Bordeaux regions, even Burgundy. Also the southern Adelaida region of Australia has many limestone rock deposits. They are all mainly marine sediments that have weathered into soils over many thousands of years.

MB: In that case, are there differences between central coast and northern California soils?

TR: In terms of soils, all of the California winegrowing regions have a variety of soils, ranging from recent alluvial river deposits and eroded sedimentary layers, to ancient calcareous and siliceous (silica-rich) seabed deposits. But Paso Robles has a thicker deposit of Monterey (Miocene and Pliocene age) and Paso Robles Formations (Pleistocene), which feature calcium carbonate-rich (limestone, calcareous shale and mudstone) and silicate-rich (shales and sandstone) rocks. Sonoma County is on a small wedge of the Pacific Plate, but it contains more of the igneous, granitic and quartz-rich rocks characteristic of the western NA plate; and Napa has more volcanic rocks, such as diabase and basalt.

Napa soils receive more annual moisture, resulting in more weathered, acid soils with higher organic matter. They have low to non-existent calcium carbonate, and typical soil pH’s of 5.0 to 7.0.

Paso Robles is drier and the soils are less highly weathered, covered by less native biomass, and generally have lower organic matter levels. They’re more alkaline, with higher soil pH’s ranging from 6.0 to 8.5.

In both Napa and Paso Robles, the northern exposures capture more rainfall in the deeper soils and have more humus and vegetative growth. The southern exposures are drier and support less vegetative cover.

In Paso Robles, the western exposures are also generally drier and warmer due to afternoon sun exposure. The eastern hillslopes are often more moist and have slightly deeper soils.

MB: How do soils in the two different regions react to rainfall and extreme temperatures?

TR: Paso Robles soils and their parent materials have relatively high water supplying capacities and good soil structures, due to their calcareous nature and coincidentally high calcium levels. Granular and blocky soil structures result in porous soils and allow for good drainage. Appropriate irrigation practices and a potential loss of nutrients through soil leaching are often a prime concern for Paso Robles grape growers.

Some alluvial soils derived from the layered Paso Robles Formation pose unique concerns. Clay may become compacted and is often washed down into basins, fans and valleys. Clay has a “plastic” quality and a high water-holding capacity. When extremely wet, it becomes like liquid plastic and it can “drown” vines because it does not allow a flow of oxygen to the roots. Essentially, in flooded conditions, the vines will suffocate.

Conversely, when expanding clays (montmorillonites) dry, they can shrink aggressively and crack. Shrinking clay has even been known to snap grape roots in half. Interestingly, that may actually be a good thing, sort of a self-pruning mechanism, particularly in old vine vineyards.

Fortunately, the interwoven calcareous soils throughout Paso Robles result in favorable water drainage. Napa has deep alluvial soils in the valley, which are good for vineyard yields, but their alluvial and clay soils rest on solid volcanic and granitic beds, which often reduce downward water drainage. Grape rootstock and scion choices in each of these areas will be entirely different due to the differences in climate and soils.

MB: What are the basic differences and similarities between the terrains of East Paso versus West Paso?

TR: Soils planted to vineyards west of the Salinas River are mainly derived from the Monterey Formation, which is composed of shale, mudstone, siltstone and limestone. They are both calcareous (carbonate-rich) and siliceous (silica-rich).

Soils planted to vineyards east of the Salinas River are derived from a wider variety of parent materials. The river bench areas are mainly alluvial sediments from the Paso Robles Formation and have varying depths of clay, gravel and sand. Compacted subsoil horizons often restrict downward water flow and limit vine root growth. However, there are also many upland hillside soil areas on the east side which are derived from the Monterey Formation and resemble the soils on the west side of the river.

There’s more homogeneity on the west side; soils on the east side are more diverse with no single dominant soil type.

MB: What is your response to the charge that there is no limestone in West Paso Robles?

TR: Limestone does exist throughout Paso Robles, but not in huge formations like boulders or cliffs, as in parts of France. On the east side, it’s usually found at depths of five feet or more, buried by the Paso Robles Formation sediments. These strongly effervescent lime layers exist closer to the water table and may take the form of a stickier subsoil. Western Paso Robles rocks are a combination of calcareous and porcelainous (silica-containing) mudstone and calcareous shale, but there’s also dolomite and other carbonate rocks. Limestone layers in west Paso Robles exist as thin one or two-foot thick ribbons, laced throughout the other calcareous soils. Because of the more rugged topography, you can often find layers of limestone on eroded ridgetops, and the boulders you see dug up for walls are often large chunks of limestone and calcareous sandstone. They are sometimes displaced and sandwiched between porcelainous sedimentary rock beds. Parts of Paso Robles have been so folded they look like overturned omelets.

MB: What are the black soils found in streaks in the eastern El Pomar district?

TR: I suspect these shallow streaks of rounded, black stones and earth are simply carbon-rich layers formed by old, but not ancient, lake beds and stream beds.

MB: A winemaker recently said that high acid, low pH soils (5.0 to 7.0) result in high acid, low pH wines. Do you know if this is true?

TR: This is a somewhat simplistic correlation. I think it is probably related more closely to levels of potassium. High acid soils are often associated with low potassium, as well as low pH, and this would probably create wines with higher acidity.

Deep clay soils, which exist in both Paso Robles and northern California, have higher moisture contents and anecdotally, there is lower acidity, lower Brix and higher moisture in the grapes. Shallow, rocky soils, which are low in potassium, result in grapes with higher acidity and higher Brix, lower moisture, and lower yields. Therefore, it’s likely that you’ll get both higher alcohol and richer flavors in wines made from grapes grown on shallow, alkaline soils.

MB: If you were going to plant a vineyard and you could choose any site in Paso Robles, where and what would you choose to plant?

TR: It would be so hard to choose! There are so many good places, and such diversity. It’s just a question of matching soil properties with appropriate rootstocks, choosing a scion to match the climate, and making wise irrigation choices. Irrigation water quality and quantity would also be major factors influencing the best vineyard location.

~ Mary Baker, Regional Correspondent – Paso Robles


To comment on Mary Baker’s writings and thoughts, contact her at m.baker@appellationamerica.com