Five thousand feet below and about 20 degrees warmer lies the city of Albuquerque in the Rio Grande basin within the Rio Grande Rift, a fault zone that has formed basins and ranges from southern Colorado into Mexico and reaches five miles deep as one of the greatest troughs on Earth. The ancient Rio Grande began invading the rift about three million years ago, and in the valley below the river is lined with the bosque forest of cottonwood, elm, salt cedar, and Russian olive trees. The Sandia Mountains were created about five million years ago when two large blocks of earth moved, with one forced upward along a fault and tilted eastward 15 degrees to form the Sandias while the other dropped downward to form the Rio Grande basin, where movement can still be detected along the fault zone. Beneath Albuquerque, fifteen thousand feet of water-deposited sand and mud fill the basin, and most of the city's water is trapped in the pore spaces of this sediment. Volcanic activity during rifting produced large quantities of volcanic rocks in the rift, including Mount Taylor, 66 miles to the west, an extinct volcano that rises nearly half a mile above its base and reaches an elevation of 11,301 feet, and the Albuquerque volcanoes, small black cones 19 miles west of the Sandia Mountains that are only 190,000 years old.