what do you cross when traveling from Mexico City to Durango?

My daughter needs to answer this question for her homework. It says that when you fly from Mexico City to Durango you will cross the Mexican ________ Help! We have looked at maps and can't figure it out.
I should clarify that it is Durango,Mexico not Colorado.

border. Durango is in Texas.

The easiest response would be "desert"....... within this ecosystem we have a place called "Bolson de Mapimi" and within this latter an area called "Zona del Silencio", but I don't think either of these 2 is what you're looking for -- way too specific for your kid's homework if you don't live in Mexico.... so I guess I'd go for the "desert" thing.

It might help to know what your daughter was currently studying.Looking at a map and drawing a straight line from Mexico City to Durango,Durango you could be flying over portions of six Mexican states. Mexico,Queretaro,Guanajato Jalisco,Aguascalintes and Zacatecas.The mountain range is the Sierra Madre Occidental.... Hope this helps you.

****You will also cross the Tropic of Cancer****
See Link:
http://en.wikipedia.org/wiki/Image:World_map_with_tropic_of_cancer.jpg

sierra madre