Fortunately, the tire went flat in the garage
That smooth surface at the bottom of the gash is the tire liner, of course.
Looking at the condition of the tire treads, however, shows we’re not getting an order of magnitude more flats from road debris, so it’s a net win.
Marathons have plenty of rubber and include a Kevlar puncture-resistant layer. In this case, that was just barely enough!
I replaced the tire liner in the front wheel; the previous tube had gone flat, as mentioned there, due to erosion from the end of the liner.
The difference: Slime liners are thinner and don’t have nearly the abrasive power.
My rule of thumb: when I can see the liner, it’s time to replace the tire.