 The climate is classified as mid-region III under the University of California, Davis system of heat summation by degree days, averaging about 3,350 degree days per year, with the mid-valley averaging 3,250 to 3,450 degree days per year, and the upper valley averaging 3,700 to 3,750 degree days.Sedimentary rock composed of sandstone and shale of the Great Valley Sequence that is Early Cretaceous in age makes up the bedrock that is buried beneath the fill of the valley floor.Similar sedimentary rocks crop out in the hills that border the east and north sides of the valley.