Symbol of Corosh'nai
From The Stargate Omnipedia
A dark and imposing statue representing a warning to "turn back." It is used as a warning to Goa'uld and Jaffa, typically posted near the Stargate, to advise travelers that the world has been left in waste by Goa'uld invaders, and is possibly still radioactive.
KEY EPISODE
There But For the Grace of God - Teal'c finds a symbol of Corosh'nai on P3R-233.