GSA Today
Volume 23, Number 10 (October 2013)

About the cover:
Laminated argillites of the Paleoproterozoic Gowganda Formation in the Huronian outcrop belt, Ontario, Canada. Fine (dark) and coarse layers are thought to represent seasonal freeze-thaw cycles, but paleomagnetic results indicate a tropical setting. Note striking occurrence of pairs of identical light layers, possibly representing two similar summer seasons in each year, as predicted under high obliquity conditions. Dropstone is about 10 cm in height. See “Evolution of Earth’s climatic system: Evidence from ice ages, isotopes, and impacts,” p. 4–10.

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