Originally Posted by xeno000
Cap's shield is made of Vibranium, which can absorb blows by deadening the vibrations they cause. When the mythical metal is struck as hard as Thor hit it, the energy is too much for it to absorb so it reflects it back outward. So when Thor put the hammer down Cap's shield absorbed as much of the energy from that blow as it could and reflected the rest, which knocked the Thunder God back and flattened the surrounding forest.
The same thing happened when Cap fought the Chitauri holding hostages later in the film. His shield not only protected him from the aliens' bomb but absorbed enough of the blast to protect the fleeing hostages as well. At least, that's my understanding of the comic book physics involved.
Ahhhh. Thanks for the info. Not as knowledgeable of Cap as Thor.