otakar has a good reference here; both for color and shape.
Been out here in Arizona for 25+ years now and have explored Monument Valley and scores of places near it from northern Arizona and southern Utah. Red rock country starts in Sedona, AZ and continues north up to Moab, UT. Also goes west to near California. Hiking and traveling around these places makes one almost feel like he/she is on Mars. Top inspirations for Terragen! Do a search on Google for the 'Grand Circle' for some scenes I'm referring to.
All of my images since I started with Terragen Classic (and TG2) nine years ago are at RDR if you care to have a look. Try to put some flat spaces between those landforms; create a sense of space. The sky is usually clear. High cirrus are around sometimes. Lower your 'gamma' setting and increase your 'red sky decay with most scenes of this type. It will enhance your image.
Bob