Rather than leaf miner, perhaps a small chewer which removed the surface cells. The exposed tissue is brown. So, whatever is responsible, is now gone.

Amazing. Used search phrase "ash leaf beetle" and that's what it is! Technical name leaf beetle is Octotoma plicatula

from forestryimages
http://www.forestryimages.org/browse/detail.cfm?imgnum=3067039

Here is a link that might be useful: ash leaf beetle damage

http://bugguide.net/node/view/509121

Here is a link that might be useful: the beetle

Good call, Jean! What a weird little beetle. I don't think I've ever seen one nor have I ever seen such damage.

Hey, thanks, Jean. Certainly an ugly little critter. 'Twas a new one to me!
rhizo - the damage is pretty extensive and highly noticeable, although I'm sure it won't hurt the tree. It will be interesting to see if it returns next year.
hortster

I rather doubt it will damage the tree, especially as it's almost time for the leaves to fall on their own.