The desert zone also accounts for about a third of the state's surface area. The Chihuahuan Desert is an international biome that also extends into the neighboring Mexican state of Coahuila and into the U.S. states of Texas and New Mexico. The desert zone is mainly of flat topography with some small mountain ranges that run north to south. The desert in the state varies slightly with a small variant in climate. The lower elevations of the desert zone are found in the north along the Rio Grande which experience hotter temperatures in the summer and winter while the southern portion of the desert zone experiences cooler temperatures due to its higher elevation. The Samalayuca dunes cover an area of about 150 km2; it is an impressive site of the Chihuahuan Desert and is a protected area by the state due to unique species of plants and animals.