Description: From Wathomas and others (2000): "Lahar deposits in the valley of West Glacier Creek downstream from Umbrella Glacier (location UG-1, Fig. 14 [in original text]) are evidence that lahars have inundated this valley at least twice in the past 2300 yr. An older lahar deposit (L1, Fig. 15 [in original text]) is a massive, matrix-supported, dark-yellow brown (10YR 3/6) diamict, containing cobble-to-boulder-size clasts of andesite and minor amounts (<20%) of hydrothermally altered rock. The majority of the clasts in the lahar deposit are subangular to angular. Ten to twenty percent of the clasts in lahar deposit L1 are rounded to subrounded and were likely entrained from the streambed as the lahar advanced downstream. A buried soil is developed on the surface of lahar L1 (Fig. 15 [in original text]) and soil organic matter from the soil yielded a radiocarbon age of 2360+/-80 yr BP (Table 3 [in original text]), a minimum-limiting age for inundation of the valley by lahar."
"Although we found no juvenile material in either lahar deposit, we interpret lahar deposit L1 as a noncohesive lahar that may have formed during an eruption of Iliamna Volcano about 2400 yr BP."