GLACIAL EROSION AND PRE-EXISTING FRACTURE NETWORKS COLLABORATE TO CREATE THE ACADIA NATIONAL PARK LANDSCAPE

Thousands of visitors flock to Acadia National Park every year to admire the breathtaking landscape reflecting a rich geologic history spanning over 500 million years of tectonic, volcanic, climatic, and glacial processes. This is part of a broad study exploring how bedrock lithology, pre-glacial fractures, Pleistocene glaciers, and modern surface processes collaborate to produce the landscape of Mt. Desert Island (MDI). There is a clear correlation between the orientation of valleys, bedrock fracture sets, and streamlined landscape features of MDI. Glaciers are often credited for the production of this iconic landscape, while the role fractures has received less attention. Previous workers have suggested that glaciers took advantage of already existing landscape lows, often following the dominant ~N-S regional fracture set. Indeed, U-shaped valleys and elongate lakes generally follow the orientation of this fracture set. Where glacial striations are preserved on the bedrock or in the orientation of streamlined landscape features, we see ice flow was generally oriented at ~150, sub-parallel to the N-S fracture set (~170) and sub-perpendicular to a second ~E-W fracture set (~100). Based on visual inspection of hillshade images generated from a regional lidar dataset, we note that the occurrence of glacial landforms is intimately related to both of these fracture sets. Here we quantify fracture orientation and fracture density from field sites across the island’s highest peak, Cadillac Mountain (1529’ a.m.s.l.). We performed a remote analysis of the region using hillshade images and topographic profiles to identify different glacial erosional landforms throughout the region of high topography. The results obtained from using both techniques suggest that the dominant ~N-S fracture set, oriented at ~160-180, likely hosted pre-glacial valleys in regions of high fracture density. As ice flow was generally sub-parallel to this fracture set, valley walls often have slightly asymmetric shapes with western walls being slightly steeper than eastern walls due to enhanced plucking on the western walls. The E-W fracture set, oriented at ~090-110, sub-perpendicular to ice flow, tends to delimit roche mountonnées and other glacial erosional landforms at all scales across the island.