Sijes is a small town which services a borate mining district. Borate beds intercalated with clays, tuff and sandstone of the Sijes Formation. In part buried under sediments washed down from the western flanks of the Sierra de Sijes.
The borate beds here constitute the world's largest deposit of hydroboracite. Ref: Journal of Sedimentary Research, Volume 70, N° 3, p 664 to 681.