Here it is, a little closer:
and here's a short video I took:
After we parked back home, I went off to see if I could help it into the long grass, but it had disappeared...whether it had successfully glided off, or had become lunch for a bird, I don't know. The snake was very alert, in a state of alarm, and definitely not gliding along; I suspect it might have been dropped by a bird, though it did not look injured.
I've been googling for snakes of Missouri, but I can't seem to get a perfect match for these markings. Now I'm sorry I didn't pick up the brochure on the snakes that I saw yesterday in the Visitors' Center at Forest Park! :)
I've asked the experts, let's see what they say.
Update:Danny Brown says it's a
MIDLAND BROWN SNAKE