Coombsville itself is a bowl-shaped depression, cradled by a crescent-shaped section of the foothills of the Vacas Range, topped by Mt. George and Atlas Peak and flanked by the town of Napa and the Napa River. Most of the area was blanketed by volcanic ash from Mt. George. Alluvial flows covered the ash with cobble-stone strewn layers of rich loams. Pockets of volcanic soils and rocks pepper the landscape as well. The gravelly loams and rocky volcanic soils drain easily and the ash sub-soils hold water, which the vines can access as the dry growing season progresses.