Pedro Pablo de Quiroga founded San Agustín on April 4, 1776, but people had lived in this valley for far longer. Since at least the sixteenth century the Cuyan territory was home to indigenous communities, the Huarpes, the Capayanes, the Olongastas, and the Yacampis, a Diaguita group from neighboring La Rioja. Their presence still marks the land around the town. Visitors can find La Piedra Pintada, the Meseta Ritual, and stone mortars worn into the rock by generations of grinding, tangible traces of the people who made a life here long before Spanish missionaries and ranchers arrived. The town remains small, its few thousand residents clustered where water and shade make the valley livable.

San Agustín's modern purpose is to serve as the doorway to Ischigualasto Provincial Park, better known as the Valle de la Luna, the Valley of the Moon. The park earned that name back in 1943, and standing among its wind-carved badlands of pale and banded rock, you understand why. Most of Ischigualasto lies within Valle Fértil Department, and in 2000 UNESCO inscribed it, together with the adjacent Talampaya National Park across the La Rioja line, as a World Heritage Site. The town's economy now runs largely on travelers passing through, people who sleep and eat here before driving out to walk on terrain that looks scoured and lunar.

What makes Ischigualasto extraordinary is not just its strangeness but its age. The rock layers preserve a near-continuous record of the Late Triassic, roughly 231 to 226 million years ago, the very window when dinosaurs first emerged. Among the most important fossils ever pulled from this ground is Eoraptor lunensis, a small, primitive dinosaur whose holotype skeleton was discovered here in 1991 by paleontologist Ricardo Martínez. The carnivorous Herrerasaurus turns up more abundantly still. Walking the park, you are reading one of the clearest pages anywhere on Earth of the moment life took the turn that would dominate the planet for the next 165 million years.

This is also country that shakes. Western Argentina's Cuyo region sits in an active seismic zone, and San Agustín's history is punctuated by it. The earthquake of January 15, 1861, was the strongest ever recorded in the country to that point, and successive local governments have written caution into their building codes ever since. The lesson was reinforced cruelly in 1944, when a quake devastated the city of San Juan, and again in 1977, when the Caucete earthquake left up to forty thousand people homeless. That later disaster produced a haunting effect called liquefaction across a vast area: sand welling up through cracks more than a meter wide, the solid ground briefly behaving like liquid. Argentina marks its Day of Civil Defense in memory of it.

For all that drama in the rock and the earth, San Agustín itself is a place of quiet. Streets laid out in a tidy grid run up against the Sierra de Valle Fértil; an artificial lake, the Embalse San Agustín, offers fishing for silverside and a place to slow down. Beyond the town wait trails to the three peaks of Valle Fértil, the ancient olive trees of La Mesada, the Jesuit ruins of Las Tumanas, and the leather, fabric, and woodcraft of nearby La Majadita. It is a town that asks little and gives a great deal: shade, water, and a doorway into deep time.