Other parts of the park consist of interbedded layers of sedimentary rock and volcanic ash. The Rio Grande forms the southern boundary of the park, and of the United States in this region. Where tectonic activity has produced mountains that lie across the path of the river, the Rio Grande has cut spectacular canyons . Santa Elena Canyon , the westernmost canyon in the park, was cut through faulted and uplifted Cretaceous limestone. The canyon walls reach a height of 1500 feet in places.
Erosion produces rock falls in the canyons.
Boquillas Canyon , the easternmost and longest canyon in the park, cuts through the Sierra del Carmen mountain range. Wind-blown sand is deposited along the side of Boquillas Canyon. Wind carrying sand can abrade and polish bedrock surfaces. Archeologists study corn-grinding pits and other traces left by prehistoric inhabitants along the Rio Grande. Yucca blooms in the desert.