The Geology of Tonto National Monument: Ancient Rock and Salado Cliff Dwellings

At Tonto National Monument in Arizona, geology shaped daily life for the ancestral people who lived there from about 1150 to 1450 CE. In the 1930s, archeologists working in the Salt River valley ("Río Salado") named this culture and its artifacts Salado. The rocks gave the Salado people raw material for tools and buildings; natural caves gave them shelter and places to build dwellings; springs supplied water; and the major streams made farming possible.

Cliff dwellings at Tonto National Monument. Image from the National Park Service's page.

Very old rocks

The oldest rocks in the basin are more than 1.6 billion years old — older than many rocks at the bottom of the Grand Canyon. Most of the park's rocks are a little younger, 1.0 to 1.6 billion years old. The monument lies in a transition zone between the Colorado Plateau and the Basin and Range, a zone that holds the most extensive exposure of Arizona's oldest rocks and gives the region its bold character. At nearly 8,000 feet, Mazatzal Peak is the highest point of these ancient rocks in the transition zone. These same rocks hold the caves and cliff dwellings of the monument.

What you can see

The park's geologic features include:

• Proterozoic rocks;
• unconformities;
• conglomerates;
• basin fill;
• Pleistocene and Holocene deposits;
• caves and cliff dwellings — including a reassignment of the Lower Cliff Dwelling to the Mescal Limestone;
• springs and groundwater;
• geothermal resources;
• lithic resources.

Geologic map of the monument, from the NPS Geologic Resources Inventory.

What park staff manage

The same geology brings a set of resource management issues:

• fire and slope movements;
• flash floods;
• vibration from aircraft;
• rockfall;
• seismicity, active faults and earthquakes;
• management of cave resources;
• inventory, monitoring and protection of fossils;
• climate change.

Part of a larger region

Tonto National Monument belongs to the southern Basin and Range physiographic province and shares its geologic history and characteristic formations with a region well beyond the park.

Sources

• National Park Service: "NPS Geodiversity Atlas—Tonto National Monument, Arizona."