I consider myself much more of a writer than a cartographer as well(hence why I'm here trying to get better) and I tend to agree with Ascension to a point. Up until recently all of my maps were hand drawn so I didn't need magic to explain an odd bit of topography. BUT I also think maps can drive story. Take the World of Greyhawk setting, The Sea of Dust which is a huge desert in the Southwestern section of the continent was created by a "massive" magical battle. Now obviously in this case the story drove the map but I could easily see it going the other way. If the designers had wanted a desert there and had to explain it's existence. It gave us two of my favorite "legends". The Rain of Colorless Fire and The Invoked Devastation. and to agree with Ascension once again, the magic used in those battles was so intense that it forever altered how magic worked on Oerth. Ultimately it's your story and your map which drives which is less important than if you and your players enjoy it and it is written in such as way as to make it believable.