Complex sentence: Gray agreed to teach for one year, which included a break from August-December 1832, causing him to cancel his plans for an expedition to Mexico, which at the time included what is now the southwestern United States.
Split into simpler sentences: Gray agreed to teach for one year, which included a break from August-December 1832. This job caused him to cancel his plans for an expedition to Mexico, which at the time included what is now the southwestern United States.