If the pig's head is a symbol of one type of power, it is the incredible power of human nature, in this case mostly for evil. The beast even reveals this side of power to Simon. The boys start to exhibit some aspects of this type of power as they give themselves over to Jack's tribe, submitting to their more carnal desires to hunt, to be anonymous through the use of paint and filth, to hurt and to kill and to feed. This power becomes irresistable to all but Ralph and Piggy and Samneric, even as some of the boys under its spell object to the use of that power by Jack.
The conch stands for the power of civilization, the power of rules and order and, in this case, the very thin veneer of goodness that it lends to people. It only takes a few days for this power to be overcome by the power of disorder, of inner desire, of the animalistic tendencies of the boys.
Through the boys, Golding has crafted a commentary on the influence of both kinds of powers and perhaps what holds it at bay some of the time in "civilized" society.
In the book The Lord of the Flies the pig's head demonstrates Jack's power and ability to hunt and obtain food for the group. The conch demonstrates civilization. Ralph gained power in the beginning of the book when he held up the conch and blew it. Because he held the conch the group voted for him to be the leader.
As the children become hungry and frightened they pull towards Jack because he is able to kill and feed them. he is also the aggressor and can control them by means of his strong and foreboding personality. The conch is destroyed which indicates that the last of civilization has been destroyed.