GEOLOGY OF MOUND DESERT ROCK AND GREAT DUCK ISLAND

Great Duck Island and Mount Desert Rock (MDR) are two remote islands off the southern coast of Mount Desert Island (MDI), Maine. Both islands are working field stations maintained by College of the Atlantic. Each island has been host to many ecological research projects during the last decade, although none have specifically focused on the bedrock and surficial geology. Maine Geological Survey maps show both islands as composed dominantly of Devonian granite, similar to nearby islands including MDI. The goal of this project was to map the dominant fractures, dikes, bedrock types as well as the glacial features and sediments of both islands and create geologic maps using the ArcGIS software. In addition, Mount Desert Rock lacks a topographic dataset and is beyond the range of available aerial satellite and Lidar data. As part of this study we began the process of building a digital elevation model using Agisoft Photoscan software in conjunction with a SLR camera and drone. Preliminary data show both Mount Desert Rock and Great Duck Island follow regional trends of dominant north-south and east-west oriented fracture sets and minor fault zones. The entire ~7 acre island of MDR comprises a fine to medium-grained light gray phaneritic granite with less than ~5% mafic minerals. Conversely, Great Duck Island comprises both a fine-grained bright pink granite, also quite low in a mafic minerals, as well as a welded rhyolite. The granite crops out along the southern half of the island while the rhyolite dominates the northern half. The contact zone is ~100m wide with the granite intruding the rhyolite or encompassing angular blocks of the rhyolite. The contact is spatially correlated with an east-west oriented fracture zone in the central portion of the island, a topographically low area bound by boulder beach berms on both coasts. The rhyolite here is reminiscent of the Cranberry Island Volcanic Series of the nearby Cranberry Islands and Mt. Desert Island.