Having justified my emergency turn off by Groundskeeper Willie I was quite surprised by the deluge when I sawzalled the galvanized pipe just before the problematic valve.
I had already scheduled the "turn on" for after the valve replacement ( you can see the new valve with the hammer drill a couple posts ago) for the following Friday. The same middle-aged valve that let me down in the house, also was now letting me down on the city end of things. A deluge of water was disappearing into a mysterious place below my slab. Fortunately, after 20 minutes of pouring water, and kind conversations with Willie's partner in this business, we hit a sweet spot and the water stopped.
I will plumb the house with PEX - a plastic tubing. This will eliminate the galvanized pipe in the house - here is a cross-section of the half-century old plumbing. Check out the iron stalagtites.