Cerro Koi

Cerro Koi is a low hill on the edge of Aregua where the sandstone has taken an unusual form: the rock is jointed into hexagonal columns, an arrangement normally associated with cooling basalt rather than sedimentary stone, and the columns lie exposed on the hillside in beds and fans. Explanations differ, with most attributing the pattern to stress and cementation in the sandstone rather than to volcanic processes, and the formation is described as occurring in only a handful of places worldwide. It was declared a natural monument in 1993 along with the neighbouring Cerro Chorori. Paths run over the hill with views to Lake Ypacarai, and Aregua itself is a pottery town.

What you actually see

• The hexagonal columns - the jointed sandstone exposed on the hillside.
• The exposed beds - the fans and faces where the pattern is clearest.
• The Ypacarai view - the lake from the top of the hill.
• Aregua below - the pottery town at the foot.

When to go

April to September in the cooler months; summer heat on the bare hill is heavy. Morning and late afternoon light show the columns best. The site is open access.

Honest expectations

The formation is modest in extent and takes finding on the hillside; signage is limited. The site is unmanaged with no facilities. Quarrying and encroachment have damaged parts of the hill. The visit takes about an hour.

Nearby

Aregua lies below; Lake Ypacarai is east.