Once part of a vast inland sea, Eastern Contra Costa County is dominated by Mt. Diablo and the foothills of the Coast Range to the west and a river and delta system to the north and east.
Mount Diablo, located in the center of Contra Costa County, is the highest and most prominent landmark in our area. It's geological history, dates back thousands of years when the mountain's central core, known as the Franciscan core, was forced upward from within the earth due to geological compression. Erosion has since reduced Mt. Diablo to its present height of 3,849 feet.
The characteristics of the early inland sea determine much of our soil composition today. The peat dirt near Bethel Island and Discovery Bay resulted from the decaying vegetation that occurred as the sea receded. The rich soils of the Brentwood area result from the erosion of Mt. Diablo and the foothills, as well as the flooding of local creeks. Oakley's sandy soils blew in from the San Joaquin and Sacramento Rivers which carried sand from the Central Valley.
Many other geological features occur in particular areas of East County. Numerous coal deposits were found in the foothills. Hot mineral and sulfur springs once existed in the Byron area, and underground deposits of natural gas and oil have been located throughout East County.
of this information was taken from Mount Diablo, The "Devil" Mountain,