What types of rocks form the Sierra Madre Oriental?

The Sierra Madre Oriental is composed primarily of folded sedimentary rocks dating to the Cretaceous Period (approximately 145 to 65 million years ago). In addition to these sedimentary formations, the range features numerous igneous intrusion forms. The current rugged terrain of the mountains is the result of uplift, faulting, and erosion that have occurred since approximately 23 million years ago.1