Symbolism, Imagery, Allegory
Snakes. Why did it have to be snakes?
Cheer up, Indy. At least you don't lose your left arm over it. But Mattie does—which means that snakes are a good candidate for being a big, fat symbol. Let's take a look her encounter with the ball of snakes after falling in the pit during the novel's climactic scene:
[…] My eyes were attracted to something—movement?—within the cavity formed by the curving gray ribs. I leaned in for a closer look. Snakes! A ball of snakes! I flung myself back but there was no real retreat for me, imprisoned as I was in the mossy trap. (7.260)
During the late eighteen hundreds, British writers like Bram Stoker (Dracula) and American writers like Edgar Allan Poe were creating tales of horror featuring characters with diseases of the body and mind. (Check out "Genre" for more about the Gothic.) Gothic stories often feature small, secret or hidden spaces filled with bats, snakes, spiders, dead bodies—you know, standard nightmare stuff. And, obviously, they're usually heavy-handed symbols on the dark mindsets of their characters and of some of the larger themes of the particular story.
Here's one idea: maybe the snake-pit symbolizes Mattie's true state of mind at this time. Underneath the tough, wise-cracking, all-business image she relies on to survive, she's a girl grieving for her father, a young woman caught up in a quest for vengeance and justice in a stony world full of stings and bites, death and danger.
And here's another: maybe the snake-pit symbolizes some of author Charles Portis' darker feeling toward the America of 1875, and maybe even toward some of his characters. Writing in the 1960s, and having interviewed major civil rights leaders and covered major civil rights events as a newspaper man, Portis would have been intensely aware of civil rights issues. The snake-pit scene could be seen as Portis' vision of an America which for people of color and Native Americans (among others) might have felt a lot like a ball of snakes curled around the skeleton of a dead white guy.