Ischigualasto is a geological formation and a natural park associated with it in the province of San Juan, north-western Argentina, near the border with Chile. The Ischigualasto Provincial Park is located in the north-east of the province and its northern border is the Talampaya National Park, in La Rioja, both of which belong to the same geological formation.
The arid badlands around the formation are known as Valle de la Luna ("Valley of the Moon") due to their rugged, otherworldly appearance. In the Carnian this area was a volcanically active floodplain dominated by rivers and had a strongly seasonal rainfall. Petrified tree trunks of Protojuniperoxylon ischigualastianus more than tall attest to a rich vegetation at that time. Fossil ferns and horsetails have also been found.
Rhynchosaurs and cynodonts are by far the predominant findings among the tetrapod fossils in the park. Dinosaurs comprise only 6% of the findings, but these include early samples of the two major lineages of dinosaurs (ornithischians and saurischians). The carnivorous archosaur Herrerasaurus is the most numerous of these dinosaur fossils. Another important putative dinosaur with primitive characteristics is Eoraptor lunensis, found in Ischigualasto in the early 1990s.