Trivia: Before the election begins, Tom says, "Herbert, you're too young to vote. Back to school." The actor who played teenaged Herbert was O.Z. Whitehead, and he was approximately 50 years old at the time of filming. Additionally, the actor who played Herbert's father, Amos, was Denver Pyle, and he was 9 years younger than Whitehead. (01:08:05)
Trivia: Liberty Valance's last poker hand is, appropriately, aces and eights. (01:30:10)
Answer: At first glance, it looks like the word "LEFT" in large, widely-spaced lettering. It's very unlikely that this would be a blocking cue, telling Jimmy Stewart to move left (which he does from that point). That's not the way blocking cues are done, for one thing, and a seasoned actor such as Jimmy Stewart wouldn't need such a cue. It's also very unlikely that it's a warning message from John Wayne who is hiding in an alley across the street, on Stewart's right. John Wayne didn't want anyone to know that he actually killed Valance, so he wouldn't alert Jimmy Stewart with any messages scrawled in the street. All hypothesis aside, it's probably just footprints in the dirt, an illusion of light and shadow.