What kept the Orcs out

When Treebeard meets the company at the end of the book he talks about the part the ents played in the fight.

In his discussion he says "and down from the North and all round the wood of Laurelindórenan, which they could not get into, thanks to the Great ones who are here.’"

Question why could the Orc's not get into Lorien.

1) Does Galadriel have the power of Melian

2) If Galadriel does not have such power, are the Orc's simply kept out by an elvish fighting force

3) I am not sure I have read other explanations in other Tolkien writings