How has the pig's head (the Lord of the Flies) defiled Simon's sacred place?
When Simon first discovers the location that you are talking about, it symbolizes all of the beauty that nature is and can be. It is peaceful, quiet, and in a sense holy to Simon. That description puts the nature in a context very similar to what writers from the Romantic time period thought about nature.
The pigs head on the pike defiles everything perfect about nature. Instead of that area being a calm sanctuary to Simon, it now represents an abomination of nature and destroys his view of paradise. It does this because it shows Simon that the beauty of nature can be destroyed by the internal evil of mankind.
Whether you believe the actual "Lord of Flies" was speaking to Simon at this point is up to you. What Simon realizes in his sacred place is that no single human is immune to the evil, barbaric nature that lies within each person. As Simon feels that creeping, he "hears" the voice, and to avoid seeing and hearing more, Simon passes out.