I must inform you that this is NOT a San Francisco garter snake. The snake in the photo (above) is a western terrestrial garter snake (Thamnophis elegans). This subspecies is called the coast garter snake and is extremely abundant at the quarry site and many other areas I have surveyed. I believe we captured just over 800 coast garter snakes at the quarry site during our study. The coloration and color patterns of the coast garter snake can be highly variable. Some individuals and populations have the turquoise belly scales as the coast garter in the photo does.
I have attached a photo (below) of a San Francisco garter snake taken in southern coastal San Mateo County to show you some of the differences. Note on the SF garter photo attached that it has a red head. Also please note the presence of a solid red stripe bordered by a solid black stripe on each side of the snake.
Feel free to contact me if you have any additional questions.
Swaim Biological Consulting (SBI)
4435 First Street, PMB # 312
Livermore, CA 94551
(editor: check out California Snakes)