Glaciation of Ugwi Yamaa Valley, Kharkhiraa Uul

Martin Bevis

Whitman College, Walla Walla, WA 99362

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Martin and a Striated Boulder_	_Glacial Valley

The glacier in the compound cirque at the head of the Ugwi Yamaa (UY) valley flowed about 9 km northeast to become a tributary to the much larger glacier in the valley of the Kharkhiraa River (KR). The moraines were deposited from after the Last Glacial Maximum (LGM) to the Little Ice Age (LIA). The maximum extent of ice in the Kharkhiraa Mountains is recorded by drift about 16 km to the east on a Tertiary erosion surface. The oldest recognizable lateral moraines of the UY glacier crown valley sides and probably record a retreat from the LGM position. It appears that one of these moraines extends into the KR valley, on top of drift deposited by the Kharkhiraa glacier during retreat from its LGM position far down the KR. At the mouth of the UY valley, the stream has cut through a 20 m high terminal moraine and 20 m of bedrock. Terminal and lateral moraines record both back and down wasting of the ice; it is difficult to determine which moraines indicate pauses in recession versus readvance. The lateral moraines have been modified by mass wasting and streams; landslides, solifluction lobes, and alluvial fans are common. Due to erosion, moraine slope is expected to decrease downvalley as moraine age increases; however, the slopes of the moraines in the UY valley show no significant trends, suggesting that they are of similar age. The diameters of five lichen species are positively correlated with distance down the UY valley on the youngest moraines. Hummocky moraines and young collapse pits suggest that Neoglacial moraines were and/or are ice-cored, like the modern terminal and lateral moraines. Striated bedrock and boulders show that the UY glacier was at least in part warm-based despite extremely low temperatures indicated by nearby permafrost. The present complex of hanging, cirque, and valley glacers is characterized by medial and lateral moraines. Active ice is retreating from a gigantic LIA ice-cored terminal moraine that shows collapse where the UY river flows through it. Fine-grained sediment in the collapsed area suggests that a lake forms on the LIA moraine when the UY river is temporarily dammed.