Small ice fields on the western cordillera northeast of Lima were expanded to three times their present size in the recent past, and the regional snow line was probably about 100 m lower than it is today. Outwash from the expanded glaciers formed deltas of silt in valley-bottom lakes. When the ice lobes retreated, the reduced outwash was trapped behind recessional moraines, and the clear meltwater infiltrated into the limestone bedrock and emerged at the heads of the deltas in spring pools. The delta surfaces then became covered with peat, and radiocarbon dates for the base of the peat (1100 ± 70 and 430 ± 70 yr B.P. for two different deltas) indicate that the maximum ice advance was older than those dates and, thus, older than the Little Ice Age of many north-temperate regions. Much older moraines date from expansion of the same local summit glaciers to even lower levels in the main valleys, which had previously been inundated by the cordilleran ice field. The cordilleran deglaciation and this expansion of local glaciers probably occurred between 12,000 and 10,000 yr ago, on the basis of slightly contradictory radiocarbon dates.
Bibliographical noteFunding Information:
Introduction to the area was provided by Ramiro Matos M. of the University of Lima and John W. Rick of Stanford University. Percy Hermoza Jeri, General Gerente de Central de Cooperativas Agrarios Com-munales y Service, Cerro de Pasco, kindly made arrangements with the cooperative station at Racra-cancha for field work in the Huatacocha Valley and the adjacent cordillera. Field assistance was provided by S. Chemicoff, B. Goldstein, M. Schwartz, and J. Stein. Grants from the National Geographic Society, the Graduate School and Office of International Programs of the University of Minnesota, and the Geological Society of America made the fieldwork possible. Radiocarbon dates from the Radiocarbon Lab of the Center for Climatic Research, University of Wisconsin-Madison, were supported by the Climate Dynamics Program, National Science Foundation under grant ATM82-19079. Critical comments on the manuscript by S. C. Porter and an anonymous reviewer are acknowledged.
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