“<Samuel puts down the journal and looks back up at you.>
While trying to figure out where the shrines reside, I came across an entry that mentioned him exploring an area that sounds very similar to Dustwallow Marsh.
He mentions hiding something of great importance there.
It's a long shot, given when this earthen roamed Azeroth, but perhaps if you were to retrace his final steps you may find something.
I'll mark the location on your map.”