Tumacácori: three Spanish missions on a stretched and broken crust

Three Spanish missions stand along the Santa Cruz River in southern Arizona, on ground whose rocks range from Jurassic granite to river sediment laid down in the last few thousand years. Tumacácori National Historical Park is best known for the missions — the only unit of the National Park Service that preserves an entire, original institutional Spanish mission landscape. Underneath them is a geological story that explains the valley the river runs through, in a region built out of mountain blocks and the basins between them.

Three missions, one valley

The park lies 29 km (18 miles) north of the U.S.–Mexico border and 69 km (43 miles) south of Tucson, in Santa Cruz County. It covers about 145 hectares (360 acres) in three separate units, each around a mission founded by Jesuits or Franciscans:

Built in the eighteenth and nineteenth centuries, the missions are among the best surviving examples of Spanish Mission architecture. They are also a record of something less comfortable: the social and political hierarchy imposed on the Native American communities already living in the valley. The site was proclaimed a national monument on September 15, 1908, and redesignated a national historical park on August 6, 1990.

A landscape pulled apart

Tumacácori lies in the Basin and Range, the physiographic province that covers much of the American Southwest. Its pattern — long mountain ranges alternating with flat-floored valleys — began to form in the Miocene, roughly 12 to 5 million years ago, when regional forces stretched the continental crust. As the crust thinned it broke along faults: some blocks rose to become ranges (geologists call them horsts), and others dropped to become basins (grabens).

The Santa Cruz River runs down one of those basins. Over the most recent part of geological time, the Quaternary, it has laid down the channel, floodplain and terrace deposits that appear in all three units, along with unconsolidated alluvium — the youngest ground in the park.

The records behind it

The National Park Service's Geologic Resources Inventory, which maps the geology of more than 270 natural-resource parks, has produced a digital geologic map and a report for Tumacácori; a Soil Resources Inventory has also been completed. Both are available from the NPS DataStore, and the geodiversity atlas — rocks, minerals, sediments, fossils, landforms and the processes that shape them — is meant to be read alongside the park's living resources, not apart from them.

Sources

Written from the National Park Service's NPS Geodiversity Atlas — Tumacácori National Historical Park, Arizona, a work of the United States government in the public domain, which draws on the Geologic Resources Inventory report for the park and on Bezy (2005) and Graham (2011).