Valley floor between the Wasatch Front of the Rockies to the East and the Promontory Range to the West.
The valley floor is 4200 feet and the mountains range from 8,000 feet to 10,000 feet ASL.
How wonderful! I just visited Colorado for the first time this summer (Telluride and surrounding areas) and these photos bring me back. It's the way the sky looks against the mountains.