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Your article was very interesting. While hiking in terrains ranging from midwestern prairies to alpine environments, I’ve seen different forms of buckling due to freezing forces. Though evaporation was given a nod in the article, it too can be a significant force to form patterned ground. In March of 2002, I walked out to the middle of Death Valley at Badwater, the lowest point in the continental United States. There were polygonal regions throughout the surface as it changed from regular soil of high salt content to pure salt. The polygons typically were 3 to 5 feet across.

Derek Wallentinsen
San Pedro, Calif.

The article states that the “concentration of radioactive helium-3 isotopes” in rocks suggests information about the history of the rocks. Insofar as I know, there is only one isotope of helium-3, and it isn’t radioactive.

Donald A. Neeper
Los Alamos, N.M.

The helium-3 isotope indeed isn’t radioactive, but it is cosmogenic. That is, it’s produced when the rock is exposed at Earth’s surface and bombarded with natural cosmic rays and so carries information about a rock’s history .—S. Perkins