Lehman Caves’ Shields

This article was originally published in The Midden – Great Basin National Park: Vol. 18, No. 1, Summer 2018.

The Parachute shield (left) and others in the Grand Palace are examples of shields with abundant calcite draperies formed by supersaturated water flowing out of the medial crack between the shield’s two plates.

NPS Photo

by Louise Hose, retired NPS geologist

Lehman Caves is an extraordinary cave with a complex history. One of its more obvious distinctions is hundreds of cave shields. An unusual cave decoration, Lehman may contain more than any other cave in the world. But, why?

The simplest answer is that the bedrock is highly fractured and shields form from water oozing out of cracks. Cave shields form from fractured rocks. In addition, Lehman is an old cave (one stalagmite has been dated as 2.2 million year old) and there’s been plenty of time for lots of calcite speleothems to grow, including shields. As to how they form, the origin of shields is still something of a puzzle. What we do know is:

Upper plate of a shield near the Exit Tunnel. The lower plate has fallen away exposing the corrugated inside of the upper plate, which is suspected to have been caused by stagnant water or “strand” lines deposited when the shield was drying out and the lower plate was intact.

NPS Photo by G. Baker

a. Actively growing shields are two, parallel plates of calcite separated by a crack thin enough to only allow capillary seepage.

b. The two plates grow outward into air-filled passages and develop concentric growth bands around their water sources, which are clearly displayed by many of the half-shields along the tour route of the cave. (Note: There is no evidence that the growth bands in shields represent annual growth as in tree rings.)

c. Active growth was along the outer edge of the plates, where CO2 is able to escape into the cave atmosphere and the water became supersaturated. (Stalactites, stalagmites, and other common cave decorations form from the same process when dripping or flowing water loses carbon dioxide and becomes super-saturated.)

d. During periods of increased water flow that leads to pressure that overwhelms the capillary seepage, gravitational water deposits stalactites and draperies around the shield edges (e.g., The Parachute).

e. In many caves, including Lehman, there appears that the internal gap between plates widens toward the shield interior. Thus, undersaturated water seeping out from the original fracture may dissolve calcite from the inside of the already established plates and then re-deposit the calcite when the water reaches the edges of the plates and CO2 escapes. This eating away from the inside activity may also weaken one plate’s attachments to the site and contribute to separation and apparent collapse of the bottom plate of many shields seen within Lehman.