Meaning behind skull at the base of a cross? I recently visited the Museum of Russian Icons on vacation and while there I came across many icons that portrayed the crucifix with a skull at the base of cross (example).

What is the significance behind the skull at the base of a crucifix?
There seems to be two interpretations of the meaning of the skull at the base of the cross, the first is more symbolic and the second is more historical:


The skull represents Adam, the first man, along with original sin. Jesus was sent to Earth to absolve us of our sins through His death. Jesus' blood is washing away our sins by flowing across the skull of Adam and that Jesus is above sin. (Source: http://ricklobs.blogspot.com/2009/03/have-you-ever-noticed-skull-at-base-of.html)
The hill that Jesus was crucified on was called Golgotha, or Skull Hill. This is where Adam was reportedly buried and the Lord was crucified where Adam lay. (Source: http://mysite.pratt.edu/~wburg/paint/p3skull.html) (Source 2: http://en.wikipedia.org/wiki/Golgota)