Anyway, it appeared that I had stumbled into an amorous couple. The snake pictured above seemed to be the aggressor, and it looked like it was pursuing the other. When I rudely came on the scene, it was the one that held its ground, while the other copperhead slithered into a nearby cavity at the base of a tree. Northern Copperheads apparently mate in both spring and fall, and fall-mating females can delay gestation for several months.
All in all, an excellent night in the dark woods.