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Co-author Grace Allison exploring cave in Tonto National Forest.

Karst Features on the Tonto National Forest

Karst features, such as caves, typically occur where rocks are dissolved away (dissolution) by weakly acidic natural waters (rain, etc.). Caves commonly form in carbonate rocks, such as limestones and dolostones. Other common karst-forming rocks include gypsum and rock salt. Many people when they think of caves think of states like Missouri, Kentucky, and Florida, which are dominated by limestones or other rocks prone to dissolution. In Arizona, karst landscapes and features occur in the state’s three physiographic regions: Colorado Plateau, Transition Zone, and the Basin and Range Province. On northern Arizona’s Colorado Plateau, the Permian Kaibab Formation, a karstic limestone, is prevalent at the surface and in the shallow subsurface. Most of the karst areas identified in Arizona occur in Pennsylvanian and Permian-age carbonates.

Soda straws on cave ceiling
Close-up of cave formations known as soda straws and highlighted in pink. The debris on the ground is from ceiling collapse. The limestone the cave formed in is ~358-299 ma (million years old) and called the Escabrosa Limestone.

National Forests in Arizona protect and manage this beautiful and unique resource on federal lands. The goal of the USDA Forest Service, National Cave and Karst Program, is to protect and maintain the biologic, geologic, mineralogic, paleontological, hydrologic, cultural, educational, scientific, and recreational values of caves and karst resources. One example of a geological value found in caves are unique formations such as Soda Straws also known as tubular stalactites. Soda straws form as water leaches slowly from the roof of the cave and deposits calcite into a ring pattern. These rings continue to build and can form straws many inches in length. It’s imperative when exploring caves to value the time (thousands of years) it takes for some of these formations to form. If we want to keep karst features alive and growing, one must have preservation in mind when enjoying a cave. Always wear gloves in a cave, never break off formations as they will not return for thousands of years and be safe. Know before you go!

Photo 2. Soda straws and column
Close up of cave formations such as a column, curtains, and soda straws. These take thousands of years to form.

The Tonto National Forest caves and karst program allows recreation at your own risk on the Forest, and this includes caves. Since the passing of the Federal Cave Resources Protection Act of 1988 (FCRPA) the Forest cannot disclose cave locations and names. For more information on caving, please contact a local grotto.

An invited post by Grace Allison and Chad Harrold of Tonto National Forest. Title photo: Grace Allison, Tonto National Forest geologist, wearing proper gear when exploring a cave.

Photo 3-active straws
Active soda straws in a cave which are ~1-2 inches in length. It takes a droplet at a time, so when you see a formation that is wet, we usually say the formation is “alive”. If you touch these formations, the oils from your hands will stop them from growing.

Some Resources

Modified from AZGS website (
Additional Information on Caves and Karst in the Forest Service: (
Local grotto information: (, ).