Copperhead Photo by John White
Timber Rattlesnake Photo by John White
Cottonmouth Photo by John White
Canebreak Rattlesnake Photo by John White
The copperhead is the only poisonous snake found across all of Maryland and Virginia. Though its bite is rarely fatal to humans, it can be fatal to a terrier who is likely to be bit several times if it finds one in a hedgerow or prowling for mice near a barn.
The good news is that copperheads are not quite as common as some people believe -- most snakes believed to be copperheads are misidentified. Nonvenomous snakes that look a bit like copper heads include eastern milk snake and the northern water snake, but these snakes have round pupils while a copperhead and other poisonous snakes have yes with vertically slitted, elliptically shaped pupils
Cottonmouths are rarely found far from water and generally only in very swampy parts. Just because you see a large black snake swimming does not mean it is a cottonmouth -- it it much more likely to be a black rat snake.
The timber rattlesnake is generally found only in the western parts of Virginia and Maryland, generally in the mountains near rocks and ledges.
The canebreak rattlesnake is quite rare, and you are not very likely to come across one. The good news is that if you do, the rattles will let you know that you have a rattlesnake and what to expect.