Arwen's body could just have decayed, and grass/the hill could have grown "over" her in that way. If a corpse is left outside for long enough, then the ground can indeed grow "over" them.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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