General—The San Joaquin Valley is that portion of the great Central Valley of California lying south of the Mokelumne River and the Delta of the San Joaquin River, rising in elevation from sea‐level at Suisun Bay to about 1,000 feet above sea‐level at its southerly border.General geology—The geologic history of the Sierra Nevada and Coast Ranges is determined from the formations exposed over their surfaces and the geologic structure of the mountain blocks. That of the Valley lying between is deduced therefrom as the geologic formations of the mountains are buried beneath later or younger sediments in the Valley. The exact knowledge of subsurface conditions is limited to that revealed through the drilling of water‐wells, the static and pumping water‐levels of these wells, the chemical character of their waters, and in petroleum exploration and we