GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 81-14
Presentation Time: 9:00 AM-5:30 PM


POTTER, Noel L.1, POTTER Jr., Noel2 and RETELLE, Michael J.1, (1)Department of Geology, Bates College, Lewiston, ME 04240, (2)Department of Earth Sciences (retired), Dickinson College, Carlisle, PA 17013,

Galena Creek Rock Glacier, Absaroka Mountains, Wyoming has been the subject of multiple studies since the 1960s, when several boulders were marked to determine movement. GCRG ice is glacigenic with 1-3 m of debris over relatively clean ice. In the late 1990s over 150 boulders were marked and surveyed for 2-3 years. In 2015 we resurveyed 144 marked boulders for what is now an 18-year record. For those marked in the 1960s, the record is 49-52 years. Velocities for the earlier and later periods are comparable. Near the mid-line and on steeper slopes velocities are nearly 1 m/yr and on lateral edges and gentle slopes are a few cm/yr. The down-valley, least active part is moving 20 cm/yr or less.

New to this study is the estimation of ablation beneath the debris over 18 years of measurement by comparing actual movement to a path predicted based on local surface slope. Ablation rates average 7.4 cm/yr across the entire glacier and are greatest (up to 29.5 cm/yr) near the head of the glacier, where debris is thinnest, and least (<5 cm/yr) at the toe, where debris is thickest. Though there is no statistically significant relationship across the entire glacier, data suggest a weak trend for decreasing ablation with increasing debris thickness up to ~1.25 m. When debris is thicker, factors beyond debris thickness may have more effect on ablation rates. A trend of faster ablation near the head of the glacier and slower ablation near the toe suggests that, with a warming climate, the glacier could be “beheaded,” with the insulated toe still intact and flowing even after ice at the head stops accumulating and disappears.

Several calculations of potential paths a single boulder might travel down the length of the glacier used measured velocities from point to point along the path. They suggest a head-to-toe travel time of around 3,000 yr. The age constraint this provides is in agreement with past radiocarbon dates of 2250±35 yr on the lateral edge of the glacier near its midpoint (Konrad, 1999).

Finally, with equipment and advice provided by UNAVCO, all survey points were recorded again with PPK GPS technology, allowing for better and faster future surveys, and a three-dimensional model of the cirque headwall above the glacier was created using LiDAR scans, which will allow for future detailed studies of erosion and debris supply from the headwall.