Ribbon Ridge is a distinct natural geological formation of eastwardly tilted marine sedimentary strata dated to the upper Eocene. The Keasey Formation, exposed on the western side of the ridge, is a laminated to massive, pale gray tuffaceous mudstone, to fine tuffaceous sandstone. The overlying Pittsburg Bluffs Formation, exposed in the central and eastern side of the ridge, is a massive to thick-bedded gray to tan weathering feldspathic litharenite with tuffaceous mudstone and sandstone. Within the region, Ribbon Ridge is unusual in the presence of only these two geological strata and the intact nature of these formations. The proposed Chehalem Mountains AVA to the north and east contains other geological formations and is altered in its areas of marine sediments by geological faults and extensive landslides. The proposed Yamhill-Carlton District AVA to the west contains other marine sedimentary strata and these are more thoroughly dissected by geological faults, uplift, and erosion.
As a consequence of its distinct geological history, the soils of Ribbon Ridge are distinct from those of adjacent proposed AVAs in several significant ways. Unlike the proposed Chehalem Mountains AVA to the north and east, the soils of Ribbon Ridge are entirely derived from marine sedimentary parent materials. Unlike the proposed Yamhill-Carlton District AVA to the west, the soils of Ribbon Ridge are finer in average texture due to their finer parent materials of very fine sandstone, siltstone, and mudstone.
Because the ridge is ancient and stable, the soils from these fine sedimentary parent materials are well weathered, and consequently are on average deeper in profile and more finely structured than soils in adjacent proposed AVAs. Soils generally exhibit good water-holding capability, but are not overly generous in nutrients, tending to restrain vine canopy vigor while maintaining good health, even in non-irrigated vineyards. Underground aquifer waters for irrigation and other large-scale uses are not readily available on Ribbon Ridge. This tends to limit excess vine growth and yet also prevents extremely dense plantings in some areas.