Black Rat Snake
The Black Rat Snake is common in Maryland. You may be startled by them sunning themselves in the middle of the towpath. An adult snake is anywhere between 4 and 7 feet. You might see their shed skins hanging from trees. Although they're big snakes, they're harmless. They're non-poisonous and much prefer rodents, birds and eggs to you.
One unusual characteristic of the Black Rat Snake is that when it senses danger, it freezes and takes on a rippled posture. This photo clearly shows that posture, it must have thought I was some sort of threat to it. Like most big and scary animals, they're usually more afraid of you than you are of it!