Glaciers reached Cape Cod, Massachusetts, in the most recent ice age about 20,000 years ago. But much harsher ice ages hit the Earth in an ancient geological interval known as "the Cryogenian Period" between 750 and 600 million years ago. A team of geologists from China and the United States now report evidence of at least three ice ages during that ancient time.
Shown is a glacial cobble from the Nantuo Formation, Guizhou Province, South China. Grooved scratch marks on this cobble were formed when it was frozen in a moving glacier and ground against the underlying pavement of the glacial valley.
"The Cryogenian Period is characterized by some of the worst glaciations in earth history. But the available age constraints are so few that geoscientists dont even know how many glaciations occurred in the Cryogenian. Now, we believe we have evidence that there were at least three Cryogenian glaciations, and there may have been more," says Shuhai Xiao of Virginia Techs Department of Geosciences.
Chuanming Zhou, a Virginia Tech geosciences post-doctoral associate, along with Xiao, Robert Tucker and Zhanxiong Peng of the Department of Earth and Planetary Sciences at Washington University of St. Louis, and Xunlai Yuan and Zhe Chen of Nanjing Institute of Geology and Paleontology, are co-authors of "New constraints on the ages of Neoproterozoic glaciations in South China," the cover story in the May 2004 issue Geology, the journal of the Geological Society of America.
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