INFLUENCES OF LATE-GLACIAL AND HOLOCENE GLACIAL ACTIVITY ON ALPINE GEOMORPHOLOGY IN THE NORTH AMERICAN CORDILLERA

Glacial activity in the Late-glacial and Holocene led to deposition of moraines and creation of rock glaciers in high-elevation cirques in the North American Cordillera. Glacier advances were not extensive, and the cirques with Late-glacial or Holocene moraine deposits are generally but not always ones still containing some glacier ice.

The most common, and usually only, glacial deposits in cirques are moraines deposited at the culmination of the Little Ice Age (LIA) in the 18^th and 19^thcenturies. These moraines are relatively intact, a few meters to tens of meters high, and often nearly bare of vegetation. In a small minority of cirques not covered by the Cordilleran Ice Sheet (CIS) there is an additional moraine loop a short distance outboard of, and often about the same size as, the LIA moraine. Radiocarbon dating of organics in till, lake sediment studies, and cosmogenic nuclide exposure dating have shown that many of these outer moraines are of Younger Dryas (YD) age. Apparently YD and LIA advances were of similar magnitude and LIA advances in most cases overran older deposits. Emerging research results from northern British Columbia suggest that YD moraines there lie significantly downvalley of LIA moraines, possibly because they were deposited during deglaciation of the CIS.

In cirques or ranges with much talus production, rock glaciers are the most common cirque deposits. In Colorado and Wyoming, rock glaciers appear to fall into two groups. In cirques with east-facing aspects, larger and younger rock glaciers retain ice cores protected by thick overlying blankets of debris, and have remained active since their formation during the early Neoglacial. In other cirques with west- to north-facing aspects, smaller and older rock glaciers lost their ice cores during early Holocene warming and have not been active since the Late-glacial, based on sediment core studies from adjacent lakes. The larger rock glaciers owe their Holocene activity to wind-drifted snow, and not mean annual or summer temperature, or winter precipitation.

Compared to the larger Pleistocene glaciers, Late-glacial and Holocene glaciers and rock glaciers had limited erosional effect on alpine landscapes. Probably some talus generated on slopes in cirques during early to mid-Holocene glacier recession was cleaned out by the LIA advances.