COSMOGENIC DATING OF MORAINES AT THE SOUTHERN END OF THE CORDILLERA BLANCA, PERU

Preservation of long glacial sequences that can be dated with cosmogenic isotopes requires exceptional circumstances, including limited ice expansion at the last glacial maximum (LGM; marine isotope stage 3 or 2) relative to pre-LGM glaciations, low boulder-erosion rates, and sufficient space for ice to spread out and form moraines that are not superimposed. These conditions were met in the Junin region of the Peruvian Andes (S 11°, W 76°), where pre-LGM piedmont glaciers flowed onto the high-altitude Junin Plain repeatedly over the past half million years, producing a sequence of moraines (JQS 20: 735-758). We believe we have found another example of old moraines preserved on a high-altitude plateau in the Peruvian Andes. The Nevado Jeulla Rajo massif (S 10°00', W 77°16') marks the southern end of the Cordillera Blanca and the Callejon de Huayllas valley in the central Peruvian Andes. Laguna Conococha and the Conococha Plain (approx. 4050 masl) border the western side of the massif, which has peak altitudes of approx. 5600 meters above sea level (masl) and currently hosts a number of small glaciers. Several sets of large lateral moraines extend onto the Conococha Plain from the Jeullesh Valley. Multiple smaller end moraines lie upvalley, closer to the active ice margin. The largest pair of lateral moraines cross-cuts another pair from Jeullesh Valley and a pair from Quenua Ragra Valley to the east. Field observations of boulder weathering (mainly granodiorite and quartzite boulders) and moraine morphology suggest that these large lateral moraines all predate the LGM and that the sampled end moraine marks the LGM ice limit in the Jeullesh Valley. We have dated boulders on the three pairs of large lateral moraines and on the lowermost end moraine loop in Jeullesh Valley using surface exposure dating with cosmogenic isotopes (¹⁰Be).