Sierra de la Peña is a small range of mountains in the southwestern part of the state of Coahuila, on the Plateau Central of northern Mexico. It is situated at latitude 25° 30′ north, between longitudes 102° 30′ and 103° west (Fig 1). This range is one of several prominent mountain units extending across southern Coahuila along the south side of a broad east-west valley known as the Parras Basin. To the southeast across Puerto La Peña is Sierra de Parras; to the northwest, about 12 miles away, are Sierra de San Lorenzo and Sierra de Texas. A broad alluvial plain north of these ranges, at the west end of the Parras Basin, is named Laguna de Mayrán. A similar but much smaller plain south of Sierra de la Peña is Laguna de Viesca. It is about 13 miles wide and is bounded on the south . . .