At the base of the cross is a scene from the beginning of Reginsmál. AS Otr (otter) catches a salmon, Loki throws a stone that kills him. For this Loki was fined and the gold that he paid eventually became the gold in Fafnir’s hoard.
In the middle of the cross, we see the horse, Grani, with the treasure chest (the head of the horse is missing). And at the top of the stone there are smith’s tools – presumably those used by Regin to fashion the sword, Gram.