TILL BENCHES MARK RECEDING LATE WISCONSINAN ICE MARGINS IN MOUNTAINOUS TERRAIN: A CASE STUDY FROM MOUNT MOOSILAUKE, NH

Surficial geologic mapping of the Mount Moosilauke 7.5’ Quadrangle plotted dozens of sub-parallel, curvilinear, flat-topped, elongate benches that contour many flanks of the mountain as revealed by lidar. About 50 to 60 benches extend from near the summit down to about 2200-ft elevation. The benches range in height from 2 to 35 ft with an average spacing of 212 linear feet. Most segments of these benches are traceable for hundreds of feet across drainages and even across the floor and walls of a cirque, Jobildunk Ravine. Wright (2019) interpreted similar features on Mount Mansfield in Vermont to be moraines. However, the benches on Mount Moosilauke are more-or-less flat-surfaced, rather than ridge forms with backslopes. Thus, glacial re-advances against the mountainsides did not create these benches. Lidar reveals similar features on slopes throughout the White Mountains.

Sturtevant (2014) and Geier (2021) used lidar to interpret these bench features on Mount Moosilauke as solifluction lobes. Some short, looped segments seen on lidar are likely the result of differential slumping against the ice margin, which receded unevenly at times. Solifluction lobes described with diagrams and photographs in the literature and features mapped by the Goldthwait’s and others in the Presidential Range have elongated upslopes, frontal ridges and central depressions, and orders of magnitude less width than even the shortest elongate segments of the benches on Mount Moosilauke. The formation of these benches requires an alternative depositional model to solifluction. Till deposited earlier on slopes above the receding ice margin was no doubt initially unstable as the ice sheet margin lowered, and till was mobilized and readily transported downslope by meteoric water to bank up against the ice and infill troughs. Concurrently, glacial debris sloughed off the ice surface and was transported by meltwater to mix with the till already there to provide additional fill to the troughs and form flat-topped or slightly inclined surfaces down slope. The sediment filling the troughs and forming these benches is characteristic of flow till. As the ice margin continued to lower and nunatak size expanded, the till benches dewatered via breaches separating the linear segments that quickly became stabilized. A search for modern analogues is underway.