The distinguishing features of Moon Mountain District are its topography, geology, climate, and soils. It lies in one of the warmer parts of Sonoma County due to its inland location. At elevations ranging from 400 to 2,200 feet (120 to 670 m) above sea level, vines grow in a variety of soils, mainly rocky and of volcanic origin, on the western slopes and ridge tops of the Mayacamas Mountains. The district's boundaries are outlined by proceeding south along the mountain ridge from the Rutherford locale down the meandering Sonoma-Napa County boundary to the Sonoma-Napa county southeast intersection; then travels southwesterly in a straight line north of Arrowhead Mountain toward the Pueblo lands of Sonoma; the boundary then proceeds northwesterly along the western foothills pass the locales of Eldridge, Glen Ellen, through the Valley of the Moon and settling in the Kenwood vicinity; finally, turning east-northeasterly it proceeds to a marked 1,483-foot peak south of Sugarloaf Ridge and then east-southeasterly in a straight line back to the Sonoma-Napa County boundary.[4]
Climate
Crosswinds from both the Pacific Ocean to the west and San Pablo Bay in the south bring cooling maritime influences to the vineyards, but by the time the winds have travelled across the valley floor they have warmed considerably. Most of the region's vineyards face southwest, subjecting them to stronger afternoon sunlight. The higher elevation vineyards places their varietals above the influence of the famous Sonoma fog. The warmer climate gives Moon Mountain District a longer growing season than other parts of Sonoma Valley, with earlier budburst in the spring and a longer hang time for the grapes in the autumn. Cold air tends to drain off the hillside vineyards into the valley, reducing the risk of damaging frosts and allowing for a long, slow maturation period. As a result, Moon Mountain District grapes have a good balance of sugars and acidity, leading to well-balanced wines.[3]
Soil
Moon Mountain's volcanic origins gave the area's winemakers cause to petition for a separate AVA title, which was approved in 2013. The region's iron-rich volcanic soils are quite distinct from the sedimentary soils of the surrounding area.[4] These red, rocky soils are thin, well-drained and therefore highly suited to viticulture. Louis M. Martini's famed Monte Rosso vineyard, a 120-year-old vineyard within the appellation, is named for these red soils.[2]