Great Sand Dunes is a high elevation desert where massive dunes stand beside alpine peaks and a desert valley. The thirty square mile main dunefield is the most impressive part of a much larger sand deposit that stretches west, south, and north. Most of the sand came from the San Juan Mountains on the far western horizon, where mountain streams washed it into the San Luis Valley. Southwesterly winds then blew, bounced, and pushed the grains toward the Sangre de Cristo Mountains, piling them into the tallest dunes in North America. The Great Sand Dunes system includes a sabkha, where sand is cemented by evaporate salts from seasonal wetlands, a sand sheet of grasslands and small migrating dunes that makes up about fifty-three percent of the sand deposit, the main dunefield of massive dunes, and the mountain watershed that supplies water for the creeks flowing around the dunes.