Copperhead + Timber Rattler
Back in Memphis, the Copperheads were common but usually not very big (2'-3' long).
I did once run across a 6' Cottonmouth while fishing a local pond. It was coiled up in the grass on the North side of the pond and I almost walked right over it. I got close enough to see the big triangular shaped head and when it saw me, it reared up about 3-1/2' tall. I swung my 7' fishing rod at it and smacked it's head and it dropped and took off into the water.
I left and didn't make a single cast. Those damned things will chase you if you get in their territory...and they are fast. They can also strike while reared up making your upper body a target.
Here in PA, the timber rattlers are mostly up in the mountains and at least they warn you with their rattle when you get close. Move slowly and deliberately and you get a warning.
Oh yeah...always wear thigh-high snake proof gaiters when tromping around in their territory. PA timber rattlers don't strike higher than that.