Snake with a bulge

We were walking to a nearby open space when we came across this garter snake, perhaps a coast garter snake (Thamnophis atratus zaxanthus) at the edge of  the trail near a perennial creek.  A big lump bulging just a couple of inches past its head told us that it had just eaten a tasty critter. We came across the snake too late to see dinner consisted of, but there are lots of mice and voles in the area.  It was sitting motionless at first, but as we watched we noticed it convulsing slowly and surely, moving dinner down the digestive tract.  We watched for about 20 minutes or so and the bulge moved a good 8 inches during this time.  In between movements, the snake stayed still, doing the work of digestion.

See the bulge just south of the stick?

Then in the wink of an eye it raced off and into the creek.  End of show.