Each location in Darksiders Genesis has a map. Initially, the only thing that appears on the map is what your character sees. This means you won't see treasures and passages until you discover them yourself.
To get the entire map of a location, you must find it. The map lies on a special altar.
Simply pick up that map and you will see the whole location, with treasures and passages marked on it.
Having a complete map is useful in exploration. This way you won't miss any of the important elements of the game.
When you find a map, the game will inform you about this.
Remember that each location has a separate map. This means that you need to find them every time you start a new chapter.
However, if you return to a location where you already found a map, it will be available.