Laguna Junín is a large lake 675 km long but only 4 m deep, located in the grass-covered altiplano at an altitude of 4,100 m, northeast of Lima. Pollen analysis of a 30-m core indicates that from about 12,000 yr ago until some time after 3,000 yr ago, about 30% of the pollen was carried in by easterly winds from the east Andean forests, which today are at least 600 m lower in elevation. Contemporaneous pollen deposition in lakes on moraines of the last glaciation west of the Junín plain was dominated by local grassland types and had little contribution from this easterly source. At some time after 3,000 yr ago (perhaps about 1,200 yr ago), the pollen influx to Laguna Junín from this distant forest source greatly diminished, perhaps because of deforestation by increased human populations.
Between about 12,000 and 24,000 yr ago in the Junín core, the sparse pollen concentration was dominated by grassland types, supplemented by Polylepis/Acaena, Alnus, and Compositae blown in from the sub-puna shrublands that may have occupied much of the east Andean slopes in place of the humid forest that occurs there today. The silty sediment in this section of the core is interpreted as outwash from piedmont glaciers that invaded the plain, especially from the cordillera to the west. These glaciers left a distinctive pattern of moraines and outwash fans, which served to dam tine lake basin.
An unconformity in the sediment covers the time from about 24,000 to 39,000 yr ago, when the lake was probably dry. Before this time, the silty sediment and the dominance of grassland and shrubland pollen types indicates an earlier interval of glaciation, recorded on the landscape by an older set of moraines and outwash fans, which initially dammed the lake basin.