Areniscas columnares del Cerro Kõi

Polygonal columns are typical of basalt, but at Cerro Kõi they appear in sandstone, which happens in very few places in the world. The rock belongs to the Asunción Group and formed from sediments left by ancient rivers, still poorly consolidated at the time. When dikes of nephelinite (a volcanic rock of the Asunción Alkaline Province) cut through them, the heat and the later cooling made the sandstone contract and crack into five- and six-sided prisms. A National University of Asunción survey of 22 geological sites in Central, Cordillera and Paraguarí gave it the highest scientific value of all.

Cerro Kõi (12 hectares) and neighbouring Cerro Chororĩ (5 hectares) were declared Natural Monuments by Law 179 of 1993, which bans any quarrying of the columnar sandstone. Since 2017 they have been the starting point of the Ñandeyvytykuera Geopark project, driven by the UNA Faculty of Exact and Natural Sciences, SENATUR, MADES and the municipality of Areguá.