Lonesome Dove, Volume 2: Comanche Moon by Larry McMurtry

WHEN SCULL REALIZED the old woman was no longer in the camp above him, he fell, for the first time, into raw panic, a kind of explosion of nerves that caused him to hop wildly around the floor of the pit, cursing and yelping out strange words; he emitted cries and bursts of language as if he were farting fear out of his mouth. He became afraid of himself; if he could have bitten himself to death at that time, he would have. He leapt on top of the mound of earth he had heaped over the three corpses and sprang at the wall of the pit several times, hoping to claw his way out of it by main force.

But it was hopeless. He could not leap out of the pit. When he exhausted himself he fell back, his eyes raw and stinging with the dirt that fell in them when he leapt at the walls of the pit.