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A winter hike on the historic Oat Hill Mine trail in Calistoga is an invigorating way to spend a few hours in the Napa Valley.

Fog was dissipating in lingering arabesques when I parked at the trail head, located near the intersection of Highway 29 and Silverado Trail.

According to local geologist Dean Enderlin, this “walk through time” received its moniker due to the fact that a portion of the trail was utilized to transport quicksilver, or mercury, for use in various gold and silver mines of the Mother Lode.

The Oat Hill Mine road was constructed in 1873 and took about 20 years to complete. Mercury mining was widespread in Napa, Sonoma and Lake counties in the 1800s, when it was used to obtain the rich metals from crushed ore, which was known as the process of amalgamation.

In the 1900s, more modern methods of extracting the precious gold and silvers from the ore were developed. The mineral, cinnabar, which is red both in color and when stroked across a tile during a geologist’s “streak test” is the chief ore of mercury and was used throughout history as a pigment in ancient China, Spain, Italy and in South American countries.

Cinnabar and, consequently mercury, can be found in most volcanic regions, which explains why there were numerous mercury mines in Napa, Sonoma and Lake counties in the past. Quicksilver, found locally, was often heated and vaporized, releasing sulfur, which, no doubt created a distinctive odor. Next, it was condensed through a progression of complex cooling. The liquid-appearing, silver-hued metal was collected into large iron, or steel vessels called flasks, which typically held 76 pounds of quicksilver.

Freight wagons hauled the cargo of heavy flasks down to the rail station in Calistoga for transport. Some portions of the Oat Hill Mine trail today have characteristic wagon-wheel marks left over from the good old days. The historic Calistoga Depot, where the transport of mercury occurred, is now California Historical Land mark number 687 and is located on Lincoln Avenue in Calistoga. It was constructed by Sam Brannan in 1868, just a year before the Transcontinental Railroad, and is also famous for being known as the second-oldest enduring railroad station in California.

The Oat Hill Mine trail is 8.3 miles long, with an elevation that eases from 400 feet at the gate, to more than 2,000 feet at the Palisades. It tracks the historic route between Calistoga and Aetna Springs Road in Pope Valley. The famous mercury mines, which are farther north, are closed to the public.

You can access the Oat Hill Mine Trail three ways: via the Calistoga Trailhead, mentioned earlier, where hiking, bicycling and horseback riding are permitted, or at the Palisades Trailhead, or at the Aetna Springs Road Trailhead. When taking the Palisades Trailhead you will park in the lot off of Highway 29, at the Robert Louis Stevenson State Park. At this access point, no horses or bicycles are allowed. The Aetna Springs Road Trailhead has very limited parking and is not always accessible due to wet weather conditions.

Not far into the Oat Hill Mine hike from Calistoga there are lovely views of the Napa Valley below as the theater of the trail reveals a new show at each turn. Dramatic flounces of pastel-green lace lichen flows from outstretched branches of the oaks along the trail.

Lace lichen (Ramalina menziesii), sometimes called Spanish Moss, is California’s state lichen and is found around much of California, both north and south, proliferating to 130 miles inland. This delicate-looking green to grey composite organism stems from algae or cyanobacteria, and its scientific classification is in the fungi kingdom.

Lace lichen and many of the nearly 1,900 species of lichens in our state, has an important place in the ecosystem. Like the quintessential canary in the mine, lichens can be seen as markers to monitor climate change and air quality. Lichens are used by the critters in the ecosystem as camouflage and bird-nesting material, and deer consume it as a food source. Currently, the extracts of lichen are studied by scientists for possible sources of antibacterial medicines.

Beyond, up the trail, are Gray pine, Douglas fir, Cypress and chaparral. The grassland habitat found farther on plays its own role in the ecosystem. It provides foraging for deer and small mammals and also creates an “anchor” for the soil to aid in erosion prevention.

While each season on the trail has its own rewards, a hike later this year, in springtime will reward you with exceptional wildflower viewing while a winter walk reveals bird-viewing opportunities for the patient observer. Vultures, scrub jays, acorn woodpeckers and various hawks, to name a few, grace the firmament this season.

The natural landscapes you will encounter on the trail include some interesting geologic features as well. Here, you will note scars from an old quarry site at almost 500 feet, revealing andesite lava. Its distinctive blue-grey, weather-beaten surfaces hide minuscule feldspar crystals within. Several hundred feet further, evidence of ancient volcanism originating from the Sonoma volcanic field is found in it rhyolite tuff, as the debris left-over from lava is called. Enderlin has made quite a detailed geologic study of the trail, with its exposed volcanic mudflows and deposits revealed up ahead on the trail, at the nearly 2,000 foot level, which you may read about on the website listed, below.

For those who have the time and inclination the view of the Palisades at the 2,000-foot level is sure to amaze with its craggy, texture and rock features that tell a story of times long past.

For more information, view the Napa County Regional Park & Open Space District’s website, napaoutdoors.org/parks/oat-hill-mine-trail/.

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