Controversy persists in western Newfoundland regarding Pleistocene, particularly Late Wisconsinan, glacial ice volumes. Independently, a set of alpine glacial deposits on the flanks of the Tablelands in Gros Morne National Park has attracted much attention but little scrutiny. In this study, cosmogenic nuclide dating of the alpine deposits places some limits on post-late glacial maximum (LGM) ice dynamics in the vicinity of the Tablelands, a plateau bounded on the northeast by Trout River Gulch. Small valleys incised into the flanks of the Tablelands are floored with a diamict that contains both till and ice-contact deposits. Rock glaciers rest on the diamict, and rock glacierization also has affected talus lining the south wall of Trout River Gulch. A small moraine rests in the Devil's Punchbowl cirque. The cirque moraine, lobate deposits below the cirque moraine, rock glaciers, and a colluvial veneer overlying the till in the small valleys have cosmogenic 36 Cl ages as old as either ca. 20 or 15 ka, depending on what erosion rate is assumed, indicating that these bodies are Late Wisconsinan in age but post-date the local LGM. Trout River Gulch was deglaciated early and perhaps did not contain active ice even at the LGM, but previous work shows that ice was streaming seaward both north of Trout River Gulch and south of the Tablelands even as the gulch lay relatively ice free.Resumé: Il subsiste des controverses dans l'Ouest de Terre-Neuve quant au volume des glaces au Pléistocène, surtout au Wisconsinien tardif. Indépendamment, un ensemble de dépôts de glaciers alpins sur les flancs des Tablelands dans le parc national Gros Morne a attiré beaucoup d'attention mais peu d'analyse. Dans cette étude, la datation de dépôts alpins ayant des nucléides cosmogéniques établit certaines contraintes sur la dynamique des glaces après le dernier maximum glaciaire aux environs des Tablelands, un plateau limité au nord-est par le ravin de la rivière Trout. Les fonds de petites vallées coupant les flancs des Tablelands sont tapissés d'un diamicton qui contient du till et des dépôts de contact glaciaire. Des glaciers rocheux reposent sur le diamicton et l'englaciation des roches a aussi affecté les talus qui tapissent le mur sud du ravin de la rivière Trout. Une petite moraine réside dans le cirque Devil's Punchbowl. La moraine de cirque, des dépôts en forme de lobes sous la moraine de cirque, des glaciers de roches et un placage de sédiments colluviaux par-dessus le till dans les petites vallées ont des âges cosmogéniques 36 Cl aussi anciens que 20 ou 15 ka, selon le taux d'érosion assumé, indiquant que ces amas datent du Wisconsinien tardif mais sont plus tardifs que le dernier maximum glaciaire local. Le ravin de la rivière Trout a été déglacé tôt et il ne contenait peut-être pas de glace active même au dernier maximum glaciaire; cependant, des études antérieures ont démontré que la glace s'écoulait vers la mer à la fois au nord du ravin de la rivière Trout et au sud des Tablelands alors même que le ravin demeurait relativemen...
Cosmogenic 3 He analyses provide a tool to infer spatial variation of cirque-glacial bedrock erosion. 3 He accumulates in bedrock exposed at the surface as a result of cosmic ray bombardment; the concentration of cosmogenic 3 He increases with exposure time as well as proximity to the surface. The Twin Sisters range, North Cascades, WA is an ideal location to use cosmogenic 3 He to infer cirque-glacial erosion depths and rates, due to the dunite bedrock and the detailed record of Holocene glaciation from the nearby Mount Baker. We used field mapping, lidar data and aerial imagery to identify bedrock fractures, glacial fluting, and terminal moraines to establish the maximum Holocene extent of the Sisters Glacier. We collected bedrock samples along glacial flow-line transects in the forefield of the Sisters Glacier, as well as several samples outside the limit of Holocene glaciation. Concentrations of cosmogenic 3 He in all samples were calculated based on 3 He/ 4 He ratios found in shielded samples from a nearby quarry because standard crushing techniques did not remove mantle-sourced 3 He from the samples, introducing a significant amount of uncertainty. Our analyses of forefield samples show decreasing exposure ages (10,500-0 yrs) and increasing depths of erosion (0.15-146 cm) with proximity to the modern glacier. The patterns in erosion rates are more difficult to discern due to larger uncertainties, but potentially show increasing rates of subglacial erosion (0.001-0.7 mm/yr) with proximity to the modern glacier. These rates are consistent with those proposed by previous studies; however, the relatively low and consistent erosion rates suggest that abrasion and/or small quarrying events (centimeters to decimeters) are the dominant mechanisms of erosion underneath the Sisters Glacier. This study helps to better understand how cirque glaciers shape mountain topography. v
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