Bolsón is the American-Spanish version of bolsa, Spanish for a large purse or pouch. The image of a colossal purse is helpful in envisioning a bolson in the landscape: an extensive, saucer-shaped basin, closed at the bottom and surrounded on all sides by mountain slopes. These pouch-like depressions occur in the arid and semiarid mountainous areas of the southwestern United States and Mexico. In such desert regions, the runoff from irregular rains drains down surrounding slopes and collects in the low-lying, unvegetated area of the bolson, creating a shallow lake. The lowest area of a bolson is called the playa, and the lakes that form there are known as playa lakes. Although some playa lakes are nearly permanent, most are ephemeral; formed during the inundations of desert rain, they evaporate in dry periods, leaving behind deposits of salt and other sediments. Over time, layers of sediment accumulate, and when dry, the playa may appear as a salt ﬂat. The bolson itself—the basin that holds the intermittent playa lakes—can be very large indeed. In the novel A Mule for the Marquesa, western writer Frank O’Rourke describes one bolson as “ten thousand miles of the high desert, cupped within the waterless mountains.” O’Rourke may be taking slight poetic license, but maybe not: a bolson of ten thousand square miles isn’t, as one scientist put it, “completely out of bounds.” Some notable bolsons include the Bolsón de Mapimí, in northern Mexico; Hueco Bolson, which extends from New Mexico through El Paso into Mexico; and the Jornada del Muerto bolson, east of Las Cruces, New Mexico. Another term for bolson is playa basin.