Lehman Caves Cave Shield Study

This article was originally published in The Midden – Great Basin National Park: Vol. 20, No. 1, Summer 2020.

Some shields are very large, up to 10 feet in diameter.

NPS Photo

by Morgan Hill, Geoscientist-in-the-Park

For years it has been estimated that nearly 300 cave shields exist within Lehman Caves, with a few other occurrences in nearby caves. However, there has not been an extensive investigation into the quantity or location of the shields within Lehman Caves. During the winter of 2020 (Jan-April), GRBA partnered with Americorps’ Geoscientist-in the-Park (GIP) program to complete an inventory of the many shields located in Lehman Caves.

Cave shield formation is still puzzling. It is hypothesized that cave shields form along joints or cracks in the ceiling, wall, and floor of the cave through the process of capillary action. Calcite-rich water under hydrostatic pressure moves through the joints and cracks within the bedrock. As this water loses carbon dioxide to the cave chamber, it precipitates calcite on either side of the crack, creating an extension of the crack. This builds plates of concentrically-layered calcite separated by a thin, hair-sized crack, called a medial crack. The result is two thin calcite plates separated by a narrow, water-filled void. Eventually the water overflows and the plates are covered with additional calcite in the form of draperies, stalactites, and more.

Morgan Hill, Geoscientist-in-the-Park, looks at one of the more than 500 cave shields in Lehman Caves.

NPS Photo

For this study, we documented all identifiable cave shields within each traversable passage of Lehman Caves. Not all shields are easily identified, and not all passages are easily traversable! Our goal was to achieve a better understanding of the quantity and distribution of shields. We noted the location of the shield, which side of the pathway it is on, and if it is located on the floor, wall, or ceiling. We took measurements of the shield (width, azimuth, and inclination), as well as any visible joints or fractures in the bedrock. Shields that were inaccessible were still counted and estimated for measurements.

In total, we identified 504 cave shields and measured 205 joints. We originally hypothesized that there is a directional correlation between joints and shields. The data showed joints in in a north-northeast distribution, while shield distribution varied throughout cave passages. Only a few rooms, such as the Music Room and Crystal Palace show a strong directional distribution of shields, while most other rooms display weak distribution. These results are not unusual, as some shields could very well have grown from the jumbled fractures in the bedrock.

Many shields show evidence of condensation corrosion.

NPS Photo

Areas of the cave with large concentrations of shields included from the Music Room to the Tom Tom Room and the Grand Palace to the Sunken Garden. Areas of the cave with no shields included the Talus Room and Gypsum Annex.

Most shields are found in parts of the cave covered in dense, thick late speleothem deposits, which made measuring joints and fractures difficult. Additionally, condensation corrosion (a dissolutional process that removes calcite from cave surfaces), has likely played a significant role in the degredation of cave shields in many areas of the cave.

Analysis of the data is ongoing as we continue to learn more about the amazing cave shields in Lehman Caves.