Thomas J. Rice, professor of soil science at California Polytechnic State University in San Luis Obispo, CA, (www.pasoterroir.com) has shown that the geologic formations in Paso Robles are similar to those in the Tuscany region of Italy, and the Rhone, Bordeaux, and Burgundy regions in France. The Paso Robles AVA has thick deposits of Monterey Formations (Miocene and Pliocene ages) and Paso Robles Formations (Pleistocene age), which feature calcium carbonate-rich (limestone, calcareous shale, and mudstone) and silicate-rich (shale and sandstone) rocks.
His research has shown that redwood forests were present in Paso Robles during the Ice Age (Pleistocene age) and that huge flood plains incised the land and were later uplifted into river terraces. Folding and faulting of the land caused deeper sedimentary rock layers to be forced underneath the plains. As a result, in the high plateaus and orchards of the El Pomar region of the Paso Robles AVA, where Burbank Ranch is located, the eroded tops of the highest points are calcareous shale and mudstone, with surrounding recent alluvium and a veneer of Paso Robles Formation (Pleistocene/ Ice Age) alluvium consisting of mixed sand, gravel, and clay covering the sedimentary rock layers.
The soil in the Paso Robles AVA is ideal for grapes.