In chapter 3 of "Night," what was the first horrifying sight that Elie at first disbelieved?

When Elie and his father are marched into Auschwitz he learns of the crematoriums and what their fate is to be.  He sees a large burning pit.  Within the pit are burning bodies.  He sees people throwing babies and small children into the fire.  Elie writes:

"Not far from us, flames, huge flames, were rising from a ditch.  Something was being burned there.  A truck drew close and unloaded its hold: small children. Babies! Yes, I did see this, with my own eyes... children thrown into the flames...  I pinched myself: was I still alive? Was I awake? How was it possible that men, women, and children were being burned and the world kept silent?  No.  All this could not be true."