The Basaltic Prisms of Santa María Regla are tall columns of basalt rock that line a ravine through which water runs from the San Antonio Dam. This ravine area was part of the Santa María Regla Hacienda and was first promoted by Alexander von Humboldt in 1803.
The walls of the canyon, called the Barranca de Alcholoya, are lined by polygonal columns between thirty and fifty meters high with five or six sides. The basalt columns were created by the slow cooling of volcanic lava. The visible columns are backed by even more polygonal basalt columns. There are two waterfalls. The higher one has its water supplemented by diversions from nearby dams. The lower one is called the Cascada de la Rosa. The canyon has been prepared by the addition of stairs, walkways and hanging bridges for easy access.