Science

Geology of the Bridge

According to geologists, the formation of Tonto Natural Bridge went through several stages of development.

Stage 1: This west side of Pine Creek was formed by a flow of lava in the form of Rhyolite, a red, coarse-grained volcanic rock. The older rock then eroded, leaving the purple quartz sandstone.

The rock layers were then lithified, tilted, faulted, and eroded.

Stage 2: This area was covered by seawater, leaving sediment of sand and mud comprised of lime deposits.

Stage 3: Following the erosion of the sedimentary layers, volcanic eruptions covered the rock layers with lava, forming a basalt cap. This formation is evident on top of the hill prior to the descent into the park.

Stage 4: Over the years, by the natural process of erosion, the basalt cap broke down and was shifted by faults, creating the narrow Pine Creek Canyon.

Stage 5: Geologists estimate that over 5,000 years ago, precipitation began seeping underground through fractures and weak points in the rock, resulting in limestone aquifers.

Springs emerged as a result of aquifers carrying the dissolved limestone and depositing calcium carbonate to form a travertine dam.

Water eroded through the travertine and ultimately formed Tonto Natural Bridge.

Travertine: Rocks From the Water

Travertine is a finely crystalline form of dissolved limestone formed by the deposition of calcium carbonate in fresh water. It is a chemical sedimentary rock derived from the evaporation of spring water rich in calcium carbonate.

As water tumbles over rocks and rain percolates through the ground, the naturally slightly-acid water dissolves out calcium carbonate from the underlying limestones. This solution eventually gathers in the aquifer that supplies the area’s springs. As the spring water emerges and comes in contact with the air, carbon dioxide is released not unlike opening a pop bottle. What goes into solution can precipitate out and calcite is forced out of solution when the water evaporates forming travertine. Calcite is a mineral having the formula CaCO3, calcium carbonate.

Travertine, a form of calcite, is mostly white when freshly deposited but turns gray upon weathering. It can also be colored red, brown, or yellow by impurities such as iron compounds. A mixture of calcium carbonate and plant life can also form travertine.

Many hot springs and geysers deposit travertines. The same process forming travertine stalactites and stalagmites can be found in abundance in caves such as Kartchner Caverns State Park in southern Arizona.

Ecology Overview

Tonto Natural Bridge State Park is located in central Arizona near Payson. It is believed to be the largest natural travertine bridge in the world. The bridge stands 183 feet high over a 400-foot long tunnel that measures 150 feet at its widest point. There are three hiking trails, a picnic area, and a group use area.

The vegetation in the park is dominated by oak (Quercus arizonica, Q. gambelii, Q. turbinella, Q. undulata, Q. chrysolepis, and Q. emoryi). Other trees and shrubs include Juniper (Juniperus deppeana and J. osteosperma), pinyon (Pinus edulis), alder (Alnus oblongifolia), hackberry (Celtis reticulata), silktassel (Garrya wrightii and G. flavecens), and sumac (Rhus trilobata and R. ovata). Prickly pear (Opuntia macrorhiza), century plant (Agave parryi), and beargrass (Nolina microcarpa) are also found in the park.

Tonto Natural Bridge State Park provides habitat for animals, insects, and birds both small and large. There are five kinds of bat (Myotis velifer, M. californicus, M. lucifugus, M. yumanensis, and Euperma maculatum) living in the park. Other mammals include bobcat (Felix rufus), cottontail (Sylvilagus sp.), black bear (Ursus americanus), coyote (Canis latrans), gray fox (Urocyon cinereoargenteus), elk (Cervus elephus), and mountain lion (Puma concolor). Frequently seen avian species include turkey vulture (Cathartes aura), several raptors (hawks, osprey, and eagles), roadrunner (Geococcyx californianus), several owls, eight different woodpeckers, four species of wren, and four species of vireo.

Like many areas of Arizona, Tonto Natural Bridge State Park is also home to non-native plants and animals. These non-native species arrive in a variety of ways; some species have been accidentally introduced and humans introduced some purposefully. Himalayan blackberry (Rubus procerus) is a tasty example of non-native vegetation found in the park.