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 (30°S 68°W), and its northern border is the Talampaya National Park, in La Rioja, both of which belong to the same geological formation.
It has an area of 603.7 km2 (233 sq mi), most of them within the Valle Fértil Department, with a minor part in the Jachal Department of San Juan, at an altitude of about 1,300 m (4,265 ft) above mean sea level. The park is part of the western border of the Pampean Hills, and it features typical desert vegetation (bushes, cacti and some trees) which covers between 10 and 20% of the area. The climate is very dry, with rainfall mostly during the summer, and temperature extremes (minimum -10 °C, maximum 45 °C). There is a constant southern wind with a speed of 20-40 km/h after noon and until the evening, sometimes accompanied by an extremely strong zonda wind.
The Ischigualasto Formation contains Late Triassic (Carnian) deposits (231.4 -225.9 million years before the present), with some of the oldest known dinosaur remains, which are the world's first with regards to quality, number and importance. It is the only place in the world where nearly all of the Triassic is represented in an undisturbed sequence of rock deposits. This allows for the study of the transition between dinosaurs and ancient mammals; research is ongoing.
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 40 m (131 ft) 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.