The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Glacial Geology of Mount Desert Island
As you travel around Mount Desert Island you can see the effects of glaciation in many places: deep valleys carved into the bedrock; boulders that have been transported far from their original locations; and a patchy veneer of glacially-derived sediments ranging from clay to boulders. Before taking a closer look at some of the glacial features on the island, we will briefly consider the Ice Age so that we can better understand the events on Mount Desert Island.
The last Ice Age began some 1.7 million years ago at the beginning of the Quaternary Period as the worldwide climate cooled by 3-6° C. Because of this cooling, more snow fell in the winter than melted during the summer. Continued accumulation and compaction of the snow formed great masses of ice, called continental glaciers, over large portions of the earth. The level of the oceans dropped because of the vast amount of water frozen in these glaciers, and the land beneath the glaciers was pushed downward by the weight of the ice. The Ice Age included several periods of cold, glacial intervals that were separated by warmer, nonglacial times (Figure 16). We now appear to be in one of the warm phases, but cooling and another expansion of continental glaciers may occur within the next few thousand years. Maine probably experienced several major glacial episodes during the Ice Age, but the last episode obliterated most of the evidence of earlier glaciations.
Here in North America, the most recent continental glacier covered most of Canada and the northern United States. This ice sheet advanced across New England from the northwest prior to 25,000 years ago, and at its maximum extent about 21,000 years ago covered all of the land area of New England, including the highest mountains. The ice expanded into the Gulf of Maine in the Atlantic Ocean and terminated on the continental shelf almost 370 miles southeast of the present Maine coast. Sand and gravel deposits on the present sea floor mark the former position of the glacier's margin in this area. The continuation of the ice margin to the southwest of the Gulf of Maine is revealed by the thick accumulations of glacial sediments that form Cape Cod, Nantucket Island, Martha's Vineyard, and Long Island. The withdrawal of the last ice sheet started about 18,000 years ago as the glacier thinned and the ice margin began to recede generally northward. The high mountains of New England protruded from the glacier between 14,000 and 12,000 years ago, and lowland areas became ice free during the next 2,000 years.
This latest period of glacial activity is called the Wisconsin glaciation, named after Wisconsin where its effects are especially prominent. Along with several earlier glaciations, it took place during the geologic time interval called the Pleistocene Epoch, the scientific term for the Ice Age. The subsequent interval is called the Holocene Epoch, which extends from 10,000 years ago to the present day. Together, the Pleistocene and Holocene Epochs make up the Quaternary Period, (Figure 1 and Figure 16). On Mount Desert Island the Wisconsin ice sheet covered the highest hills and modified the earlier landscape through glacial erosion. The thin veneer of materials left as the glacier retreated is collectively called drift. On the surficial geologic map (pdf format), deposits are prefixed either with a "W", indicating deposition during the Wisconsin glaciation, or an "H", indicating deposition in the modern Holocene Epoch. The next two sections describe the processes that occurred during these intervals and examine their effects on Mount Desert Island.
Last updated on January 11, 2008