At its core, the nature of Lake County is explained by its wide spectrum of geologic formations. In places, the monumental forces of colliding continents have exposed deep ocean floor rock where serpentine is found. Those same forces have caused folding and uplift of Great Valley Sequence (characteristic of California’s Central Valley), resulting in sandstone and shale. And right under these formations, volcanic processes have broken through the surface and laid down a new landscape.
The viticultural soils of Lake County can be grouped into four broad categories: hillside soils formed on volcanic materials, hillside soils formed on sandstone and shale, terrace alluvial soils, and valley alluvial soils.
Aerial map courtesy of Google.com
Volcanic hillside soils dominate in the Red Hills and the High Valley AVAs. These soils are characteristically gravelly or rocky and very well-drained. Examples include Aiken, which formed on basalt; Arrowhead, which formed on obsidian; and Soda Bay, which formed on scoria, the porous red gravel used in landscaping. Lake County hillside volcanic soils are red, a color often associated with old, highly weathered soils. However, the Clear Lake volcanics are recent in geologic origin. The scoria formations near High Valley and the lower arm of Clear Lake are as young as 10,000 years.
Hillside soils formed on sandstone and shale are found on mountains thrust up from an ancient sea floor. These soils are well-drained and frequently shallow, with underlying sandstone found at a depth of two to four feet. These soils are yellowish-brown in color and well-suited to red grape varieties. Common soil types include Maymen, Mayacama, and Hopland.
Alluvial terraces were formed by a combination of geologic uplift and river down-cutting, resulting in a relatively level and well-drained landscape. These terraces are found throughout Lake County, including Kelsey Bench, circling valleys and Clear Lake. Many of these soils have weathered to a deep reddish color, leaving a lean soil well-suited to wine growing. Some of the Lake County soils formed on alluvial terraces include Jafa, Forbesville, and Manzanita.
Mountain valleys around Clear Lake, including Big Valley District, and proposed Upper Lake Valley which includes Middle Creek Valley, Clover Valley, and Bachelor Valley, and Scotts Valley, are level with deep alluvial deposits. The soils often have layers with differing gravel content and amounts of sand, silt, and clay, created by the movement of stream courses in the past and the changing shoreline of Clear Lake. Well-drained alluvial soils found in these areas include Still, and Lupoyoma loams, which produce some of Lake County’s famous Sauvignon Blancs.
The USDA Natural Resources Conservation Service maintains a soil survey of all the soils in the nation. To find the soil type(s) for a specific parcel, go to the Web Soil Survey (WSS). Click on the big green button to begin (Start WSS). Select state and county, click View. Then, using the tools in the upper left section of the map area, zoom in or out to navigate to the parcel, then click the AOI tool to select the specific parcel. The results will identify your Area of Interest with blue hash marks; click the Soil Map tab at the top to view the various soil types by number. To the left of the map, a list will show each soil type by name and percent. Click on each soil type for more information.