Mesa Verde's cliff-dwelling rocks were formed from shale, sandstone, coal, and siltstone laid down as marine and shoreline sediments during the late Cretaceous period about 90 million years ago, when much of North America, including southwest Colorado and the present Rocky Mountains, lay beneath a shallow inland sea. Within the last 65 million years, a series of uplifts formed the Colorado Plateau and Rocky Mountains and gradually raised the land thousands of feet above sea level, while erosion likely kept pace as streams flowed south and west from the rising La Plata and San Juan Mountains. Those ancient streams cut deep canyons across Mesa Verde's gentle slopes, dividing it into smaller fingerlike mesas on the park's southern end. Over millions of years, the Mesa Verde formations were lifted nearly 8,000 feet and gently tilted south, while the same sedimentary rocks eroded from the uplifted mountains left a 1,500-foot escarpment between Mesa Verde and the La Plata Mountains. Natural indentations created by erosion were later enlarged by Ancestral Puebloans and used as hand-and-toe-hold trails.