In Lord of the Flies, what does the boulder that kills Piggy symbolize?
Early in his novel, William Golding introduces the motif of stones and rocks. In Chapter One, for instance, as the boys discover the island, the perceive
a great platform of pink granite thrust up uncompromisingly through forest and terrace and sand and lagoon....
In fact, the pink granite is on several cliffs. On one summit, the boys heave the rocks until they smash a hole in the canopy of the forest. This act foreshadows the toppling of the huge rock that kills Piggy. Then, in Chapter Four, Golding as narrator alludes to the stone, "that token of preposterous time" that the sadistic Roger throws around little Henry who plays in the lagoon. At this time, he does not hit Henry because his arm has been conditioned by a civilization "that knew nothing of him and was in ruins."
But, once Roger is liberated from this "shame and self-consciousness" by the savage mask of face paint that Jack creates along with the hunt, he becomes impervious to any rules of conduct and frees his darker side. After one hunt, Roger plays the pig, grunting and charging at Jack. And, as the hunters descend more and more into savagery, Roger becomes more and more sadistic until he begins to terrorize Sam'n'Eric by flinging a stone between them:
They parted and Sam only just kept his footing. Some source of power began to pulse in Roger's body.
As the boys descend into savagery, there are fewer and fewer of the boys are on the side of Ralph and Piggy. Aggression begins from Roger's side. In Chapter Eleven, Roger is at the top of the mountain:
High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever....He was aware of a jolt in the earth that came to him through the soles of his feet...He obeyed an instinct that he did not know he possessed ....
and Piggy's body is hurled into the ocean. After the boys' yelling ceases, Roger "advances upon them as one wielding a nameless authority," the authority of savage power represented by the large pink granite boulder.