"Potrero" in Spanish means corral, and here it refers to a type of oval-shaped valley that is entirely encircled by steep mountains except for a very narrow and spectacularly deep canyon that offers drainage from within. Although steep walls abound along the periphery of the Potrero, convenience of access and rock quality issues have directed route development mostly within or close to the canyon (known as the "Boca" or mouth of the Potrero).

This unusual topography was formed when the top of a great dome-shaped fold of sedimentary rock was worn through near its highest point, exposing softer layers underneath that quickly eroded away. The ceiling of the fold then collapsed due to undercutting, and the debris was carried away by water escaping through a weakness in the side, eventually forming the canyon. Thus, the softer rocks found within the Potrero are actually much older than the limestone shell we climb on. This also explains why the limestone layers in the canyon are nearly vertical - they form the side walls of the dome. Each layer varies in hardness, and erosion has consequently produced the great ribs of good rock that are seen within the canyon and that (incidentally) greatly increase the total climbable area of the walls. The great arch of the collapsed fold can still be traced when looking at the walls of either of the two high peaks at the ends of the Potrero. Walk in one day and check it out!

"Chico" in Spanish means little, and while Potrero Chico is not that small (10km along the long axis), two other, larger Potreros exist in its immediate vicinity: the much larger Potrero Grande whose canyon can be seen across the valley from Hidalgo, and the triangular Potrero Garcia, which sits next to Chico back-to-back, with its canyon facing the opposite way - south towards the town of Garcia. This latter Potrero can be reached via a stout hike from Potrero Chico (see Hiking section).