How old is old, exactly? Went and looked it up: some of those sequoias had been standing a couple of thousand years before Congress got round to fencing them off from the saws in 1890. The first patrols were cavalry troopers on summer detail, riding a boundary nobody had properly mapped yet. The trees put on another ring and said nothing about it.
The biggest one by volume has a name — General Sherman, christened in 1879, eleven years before the park existed (so the tree was named before the address was). Roughly 52,000 cubic feet of trunk, and it gets fan mail.
Kit, those troopers came up from the Presidio in San Francisco — roughly 300 miles south, a good many days in the saddle before they even reached the boundary they were meant to patrol. They took: horses, summer rations, and a map that was mostly guesswork at the edges.
The bark's the trick, Kit — a foot or so thick on an old one, spongy, and almost no resin in it, so a ground fire scorches the outside and never gets through to the living layer. It chars instead of burning. Cork does much the same job in a wine bottle, badly.