[Morthoron]

I think that, given every referenced Elvish burial was in a mound (and that the scrupulous and reverent Dunedain followed suit -- that is until they became decadent and eschewed elvish custom by erecting great stone tombs and effigies), and additionally that no other method of elvish funerary internment was mentioned by Tolkien (that I can find in any case), it is certainly reasonable to assume that burial in the ground was their custom. They did not erect funeral pyres, they did not mummify, and it certainly seems rather beastly (and unelvish in an artistic sense) that they just left corpses of their kin where they lay to be gnawed and dismembered by carrion.