Geologic History

By Timothy S. Cleath, PG 3675

Located southeast of San Luis Obispo, the Edna Valley is a coastal valley formed by tectonic uplift and separation since the Miocene Epoch when the valley was a part of the sea floor and the coastline. The seven volcanic plugs (volcanic landforms created when magma hardens within a vent on an active volcano) known as the “Seven Sisters” volcanic plugs were intruded and volcanic ash was deposited on the Mesozoic high pressure-low temperature metamorphic rock (the “Franciscan assemblage”) that had been a part of a subduction zone during the Cretaceous Era. The last outcropping of the volcanic dacite rock plugs, just east of Islay Hill, is found on the Pacifc Vineyard property.

The deeper sea floor areas were filled with oil-bearing shale and diatomite as the area subsided. These fine-grained beds were overlain by coarser beach deposits when the ocean shallowed. Many clams, pectens and oysters lived and died on this beach- so much so that more than 60 feet of shells are found when drilling into these deposits. These marine deposits are known as the Pismo Formation.

From 2 million years ago to the present, the Los Osos Valley fault has been active along the southern boundary of the valley, uplifting the San Luis Range southwest of the valley and separating this area from the ocean. The West Huasna right-lateral fault zone northeast of the valley has uplifted the marine sedimentary beds, serpentinite and Franciscan rock into ridges reaching an elevation of more than 2,000 feet above sea level forming the northeastern edge of the valley. The valley area became a part of the continental landmass, and terrestrial sedimentary beds of the Paso Robles Formation were laid on top of the marine sand and shale layers from detritus eroded off of hills to the northeast.

During the last 10,000 years, the West and East Corral de Piedra and Verde Canyon creeks have flowed across Edna Valley and come together becoming Pismo Creek in Price Canyon. These creeks incised into the Paso Robles Formation during earlier wet periods followed by deposition of more than 60 feet of alluvial sediments derived from the hills to the east. Redwood pieces have been found in these alluvial sediments giving evidence of the wetter climate.

These geologic coastal processes and tectonic forces are still at work molding and forming the landscape of the Edna Valley that is open to the mild coastal breezes from Morro Bay and separated from the climatic extremes of the Paso Robles Basin.