How to get a map?
During the Mission, a character named chick Mathews will appear. To find it, you will first need to ask a stranger about it. Then Arthur will notice. that the person standing behind will jump on the horse and start running away from the main character. Morgan's task is to catch up with him using a lasso and make him give him the map.
The map will indicate that the treasure is located near Heartland Overflow. Arthur will only need to get to this place. He will see a huge tree with a cavity inside. It will contain the treasure.