LATE WISCONSIN GLACIAL HISTORY OF THE NORTHERN ALASKA PENINSULA, SOUTHWESTERN ALASKA, USA

The late Wisconsin Brooks Lake glaciation of the Alaska Peninsula incl udes five previously recognized stades, indicating considerable climat e variability during the last glaciation. During the early phase of th e Brooks Lake glaciation, an outlet lobe of the Cordilleran Ice Sheet advanced westward across a low topographic divide into the Iliamna val ley where it merged with ice from local mountain sources. During the e arliest (Kvichak) stade, which commenced sometime after 26,150 +/- 300 C-14 yr BP, ice filled the Iliamna valley and deposited the type Kvic hak moraine. Following retreat into the Iliamna Lake basin, the outlet glacier readvanced to the Iliamna-stade moraine. Subsequently, and so metime prior to 12,600 C-14 yr BP, ice retreated from Iliamna valley. This two-fold sequence of moraines deposited during the early phase of the Brooks Lake glaciation is not present in the Naknek valley, 80 km south of Iliamna valley, where high mountains diverted the ice sheet while supporting a confluent system of local-mountain glaciers. These glaciers deposited the moraine enclosing Naknek Lake, which marks the outer limit of late Wisconsin ice in the valley. Apparently, differing ice sources and glacier configurations caused the two glacial systems to respond differently to climate change or glacier-bed dynamics, or to be influenced by different climate changes. During the late phase o f the Brooks Lake glaciation, valley glaciers derived from local-mount ain sources terminated well behind the mountain fronts and deposited t he Newhalen, Iliuk, and Ukak moraines. Equilibrium-line altitude (ELA) over the Naknek valley was lowered 250-650 m below the present during the Iliuk stade. Full-glacial ELA was probably just a few tens of met ers lower, but generated extensive advance over broad, relatively flat troughs. Slope angles measured on the Kvichak and Iliamna moraines, a nd the correlative Naknek moraine are less steep (similar to 11-15 deg rees) than those of younger moraines (similar to 18-20 degrees), sugge sting that a substantial length of time separated the Iliamna and Newh alen stades. This interstadial may correlate with an interval of restr icted ice extent separating early and late phases of the late Wisconsi n recognized elsewhere in Alaska around 14,000 yr BP.