We’re back to civilization, so I can finally post the answer to the puzzle. I’ve been without cell service and our only computer access was dial-up at a glacial 37.2 kbps. Yes, kbps.
Back to our puzzle. Here’s a larger version of Friday’s photo:
It is bark, as many astute readers pointed out. As far as I know, it’s a London plane tree (Platanus), but given the promiscuity of the genus, who knows exactly what species or hybrid it is?
Other tree species were suggested by others and I immediately Googled them to see what their bark looked like. Check out Pinus bungeana (thanks to @Garden Hoe), Parrotia (from Deirdre), Smilax bona-nox (Bryn), Stewartia (one of Deb’s hedged bets), and Corymbia maculata (from Jimbo).
And now, thanks to Ed, I will forever look for pictures hidden in bark. Someone should make a “Gardener’s Rorshach test” from variegated bark!