I Exert for Dirt


My husband Nick is somewhat of an amateur geologist, a rock hound of sorts. As anyone who has visited our vineyard probably knows, if you query him about soil he will be more than happy to bend your ear about the interesting geologic events that led to the formation of the Coastal Ranges of California. He will commence the dissertation by outlining the mechanics of plate tectonics: how 250 million years ago the North American Plate and the Pacific Plate were on a collision course with each other and the Pacific Plate subducted, or slipped beneath the Continental Plate. Along this zone of slippage, the San Andreas Fault, the folding of the sea floor resulted in the Coast Range, which is composed of the crushed, crumpled and folded sea floor sediments. Then he would tell you about how about 30 million years ago the relative movements of the North American Plate and the Pacific Plate changed from a head-on contact to lateral slippage against each other. This movement continues at a rate of 37 millimeters per year which means that someday San Francisco will be alongside the vineyard. That will certainly improve our dining options.
I also learned from Nick that the oldest bedrock is called Franciscan complex and is found in the hills and mountains in the western part of Sonoma County. Franciscan is a mixture of different kinds of rock – marine sedimentary, igneous and metamorphic – formed at the time of the subduction. This mixture is called ‘melange’ and can be particularly found along the Sonoma Coast’s high ridges with the blocks of greenstone and chert swimming in a matrix of crushed Franciscan rocks. There is Franciscan complex underlying our property at lower elevations, bordering the edges of where we have planted our vineyard. The vineyard itself is planted on a cap of much younger soil– an ocean floor about 2 to 5 million years old that makes this hill where our vines grow. This layer of marine sediments capping the hills, ridges and spurs in our corner of the Sonoma Coast is a geological feature known as the Ohlson Ranch Formation.

The Ohlson Ranch Formation’s namesake is a ranch that can be viewed directly southeast from our front porch. The original homesteaders of that ranch along with the homesteaders of our property, the Petersons, were among the first to settle and farm the area over one hundred years ago. So, naturally, when we planted the vineyard I was curious about this geological feature named after our neighbors. Being analytically inclined, I delved deep into the electronic labyrinth of the Melvyl catalog and I came up with a scientific dissertation by Charles Higgins of my alma mater U.C. Davis in 1961 titled: Ohlson Ranch Formation, Pliocene, Northwestern Sonoma County, California. Bingo! I requested it and received a very old photocopied report, held together with old-school brass prong fasteners, from the U.C. Berkeley Department of Paleontology.

Higgins describes the Ohlson Ranch Formation as consisting of marine sandstone, siltstone and conglomerate up to 160 feet thick and occurring only in the vicinity of the town of Annapolis in the locality of the Ernest Ohlson Ranch. It is here, resting atop ridges composed of dipping marine strata of the Franciscan Formation, that flat-lying beds of Pliocene age fluvial sediments occur. Wave terraces and seastacks buried beneath the formation indicate that it was formed in a shallow water embayment or an inland ocean. Since the deposition of this sediment, the shallow basin had been pushed up by pressures along the San Andreas Fault or uplifted up to elevations 1000 feet during the Pliocene age and dissected by erosion, leaving isolated hill tops like ours.

Wow, ancient inland seas? Pliocene uplift? I had “Land Before Time” visions playing in my head. Was my husband’s affinity to geology geekiness rubbing off on me? The second section of this research paper is named: Paleontology of the Ohlson Ranch Formation. Unlike my 2-year-old son who has a dinosaur obsession, I was only mildly interested in dinosaurs as a kid. But the prospect of fossils in our area really intrigued me. Higgins described finding in the coarse sandstones fossils of various mollusks and even fish teeth. The most common findings were brachiopods, more commonly known as lamp shells. The greatest diversity in the findings was among the pelecypods, which included samples of scallops, cockles, whelks and clams. The fossils were cataloged along with their location and picture. Not exactly dinosaur bones, but I found this pretty exciting since it told me the story of the ancient history of our land.

I wanted to know more. This was years ago, before Facebook, so I couldn’t exactly search for old Professor Higgins, friend him and tell him “what’s on my mind” to pick his brain about what he remembered about a formation he studied over 40 years ago. So I hunted and pecked around some more and came upon a researcher here at our very own Sonoma State University geology department who not only studies soil but is very interested in wine and the concept of the terroir of Sonoma County, Dr. Terry Wright. After a month of email correspondence it was decided that Dr. Wright would make the drive out to the far west Sonoma Coast to tramp around our vineyard and talk dirt. Dr. Wright is an amiable fellow; he seemed as he would be equally comfortable at a Grateful Dead concert as in a lecture hall at the university. I suggested we take a walk through the vineyard, but he made a beeline to the pond where we had excavated to build a reservoir. There along the exposed wall of soil layers he found impressions– or external mold fossils– of shells. Excited at our find, I tried to chisel out and pick up the piece of earth that contained the shell impression only to find that the silty sandstone around it crumbled. We would have to try to make casts in order to preserve the fossils we found.

Back in our kitchen we shared some wine with Dr. Wright and talked. I asked him about Higgins’ comparison of the soil composition of the Ohlson Ranch Formation to the Merced Formation (which was later renamed the Wilson Grove Formation.) Higgins states that the formations are similar in appearance and seem to have been deposited at the same time under very similar conditions. Dr. Wright expounded as he does in his paper titled: Geology, soils and wine quality in Sonoma County, California, “The sandy loam soils of the Gold Ridge-Sebastopol series form as a direct result of breakdown of rocks of the Wilson Grove Formation. The low ridge running from Forestville to Sebastopol and south to Cotati is the classic terroir of this association, now being recognized as prime land and climate for Pinot Noir and Chardonnay grape varietals.” And then sometime after his visit to our vineyard he amended his research paper to state: “Similar rocks and soils occur in the northwestern part of Sonoma County, capping ridges north to Annapolis and providing sandy soils for high Sonoma Coast vineyard sites, prime land for Pinot Noir (Peay and Annapolis vineyards.)” Wow, is geology cool or what?

Thrilling as it may seem, I am not about to quit my day job as a winemaker to pursue geology. I was very excited, though, to learn more about what makes our land interesting and perhaps unique. We named one of our Pinot noir cuvées “Scallop Shelf” because I feel the name really reflects the sense of place of the wine, its character of elegance and finesse. There is a Kashaya Pomo Indian place name for this area which is meyécci batiwal li which means ‘where scallops lie.’ I am not sure if this was their name because of the shellfish they foraged in the coastal waters or they were referring to the same fossils in the ancient marine terraces we, too, find centuries later. So when you have our wine, think of that poor scallop who was no doubt minding his own prehistoric business, filter-feeding at the bottom of an ancient inland sea when the next thing he knew there was an earthquake and he lay at the top of a ridge. He probably wasn’t delighted– but we hope you are.

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