Counting Cave Shields in Lehman Caves

First published in The Midden*, Great Basin National Park's newsletter, Vol. 20, No. 1, Summer 2020. By Morgan Hill, Geoscientist-in-the-Park.*

Some shields are very large — up to 10 feet across. NPS photo.

For years, Lehman Caves was estimated to hold nearly 300 cave shields, with a few more in nearby caves — but no one had carefully counted or mapped them. From January to April 2020, Great Basin National Park worked with AmeriCorps' Geoscientist-in-the-Park programme to inventory them.

How shields form

It is still a puzzle, but the leading idea:

1. Shields grow along joints or cracks in the ceiling, walls or floor, by capillary action.
2. Calcite-rich water under hydrostatic pressure seeps through the cracks in the bedrock.
3. As the water loses carbon dioxide to the cave air, it deposits calcite on either side of the crack, extending it.
4. That builds two plates of concentrically layered calcite, separated by a hair-thin medial crack filled with water.
5. Eventually the water overflows, coating the plates with more calcite — draperies, stalactites and more.

Morgan Hill, Geoscientist-in-the-Park, examines one of the more than 500 cave shields in Lehman Caves. NPS photo.

The survey

The team recorded every identifiable shield in every passage they could get through — not every shield is easy to spot, and not every passage easy to travel. For each shield they noted its location, which side of the path it was on, and whether it was on the floor, wall or ceiling, and measured its width, azimuth (compass direction) and inclination, along with any visible joints or fractures in the bedrock. Shields out of reach were still counted, with estimated measurements.

What they found

• 504 cave shields and 205 joints.
• The team expected shields to line up with the joints. The joints ran mostly north-northeast, but shield directions varied from passage to passage. Only a few rooms, such as the Music Room and Crystal Palace, showed a strong direction; most showed a weak one. That is not unusual — some shields may have grown from the cave's jumbled fractures.
• Most shields: from the Music Room to the Tom Tom Room, and from the Grand Palace to the Sunken Garden. None: the Talus Room and the Gypsum Annex.
• Most shields sit in parts of the cave thickly coated with later formations, which made joints hard to measure.
• Condensation corrosion — water condensing on the rock and dissolving calcite from cave surfaces — has probably worn down shields in many parts of the cave.

Many shields show signs of condensation corrosion. NPS photo.

Analysis continues as the park learns more about the cave's shields.

Sources

• National Park Service, Great Basin National Park, "Lehman Caves Cave Shield Study," The Midden, Vol. 20, No. 1, Summer 2020. https://www.nps.gov/articles/000/lehman-caves-cave-shield-study.htm
• Words on this page from people and organisations outside the federal government are paraphrased; rewritten in hubnx's own words.