The Great Unknown of the Rio Grande Guide Book by Luis F. Aulbach focuses on an 80 mile stretch of the Rio Grande River below Santa Elena Canyon. This stretch of river is little used and has few rapids, but the scenery is spectacular. The Black Dike, a volcanic formation sill extruding out of Dominguez Mountain, cuts across the river soon after it leaves the civilized country around Castolon and Santa Elena, Mexico. The Rio Grande River then begins a desert journey toward the southernmost point of the Big Bend in a high-banked channel that is trimmed and edged in carrizo cane. Then, nearing the "tip" of the Big Bend, you enter Mariscal Canyon which is a short but interesting "big" canyon. The Great Unknown of the Rio Grande Guide Book gives mile by mile descriptions of areas along the Rio Grande River to help guide you on your next Rio Grande adventure. From camping sites, fishing tips, to detailed river information, The Great Unknown of the Rio Grande Guide Book covers everything there is to know about the Rio Grande River.