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Maine's Ice Age Trail
Maine has a remarkable legacy from the most recent Ice Age, when a vast glacial ice sheet advanced across the state from Canada. As the glacier retreated, the Atlantic Ocean lapped against the edge of the ice sheet and flooded low areas extending far inland across the southern half of the state. This meeting of ice and sea produced a distinctive landscape that is unique in the eastern United States.
Geologists have known since the mid-1800's that glaciers existed in Maine, though much remains to be learned about the causes of ice ages and the interactions of glaciers, world climate, and the oceans. Scientists continue to map and study the sedimentary deposits left by the ice sheets in Maine, not only because of academic curiosity, but also because these accumulations of sand, gravel, and other materials are important to the economy and planning for the best uses of our natural resources.
Investigations of Maine's glacial geology have revealed that the eastern part of the coastal zone - from Acadia National Park to the New Brunswick border - contains many of the best glacial features in the state. An added bonus is that much of this "Down East" region has large expanses of commercial blueberry fields, opening up broad vistas that make the glacial landforms easy to see. But the question has often arisen - How can the scientific community inform the public about the Ice Age story?
This project began when a Maine resident - Pamela Person - discovered that Wisconsin likewise has exceptional glacial features which are the focus of a geotourism route called the "Ice Age National Scenic Trail." Ms. Person traveled the Wisconsin trail and realized that coastal Maine is ideally suited for a similar educational opportunity. She brought this concept to the attention of Professor Harold Borns at the University of Maine's Climate Change Institute, and they partnered with the Maine Geological Survey and other State agencies to develop what has now become "Maine's Ice Age Trail - Down East."
With financial support from a grant by the National Science Foundation, the Trail committee produced a map of our Ice Age Trail that is now available to the public. Visitors to eastern Maine can use the map as a guide to 46 stops where they can see the effects of glacial action and sea-level change. Directions and photos on the map help locate stops along the trail, and eventually there will also be sign posts to help pinpoint the best viewpoint for each location. The map can be obtained from the University of Maine bookstore.
The Climate Change Institute at the University of Maine maintains the official website for the Ice Age Trail, where you can take a "virtual tour." Further support and publicity for Maine's Ice Age Trail were generously provided by U-Haul International, who produced a special graphic featuring the trail. You may see this on one of the 1200 trucks representing the State of Maine in their "Venture Across America" series (Figure 1). Details and links to intriguing natural history subjects in Maine and other states are available at the Venture Across America website.
The following text was originally written for the Ice Age Trail map. It provides some geological background to help understand the influence of glaciers in Maine, along with selected photos illustrating the kinds of glacial landforms seen along the trail.
Geological Themes and Highlights of the Ice Age Trail
Following the Ice Age Trail
This website complements the published map of the trail. The following sections provide background information about the Ice Age and how it affected Maine. We will also describe some of the glacial features for which the area is noted, with photos and references to stops on the Trail.
If you drive the Trail, please note that much of it crosses rural areas. There are few stores, gas stations, or motels, except in the larger towns as indicated on the map. However, the distances covered are not large, and you can visit many interesting sites along the Ice Age Trail in a day or two. The "trail blazers" have worked with landowners and the Maine Department of Transportation in an effort to pick stops with safe parking. A roadside marker will eventually be posted at each stop to help you find it. However, most turnouts are not large enough to accommodate many vehicles at one time. Drivers should be alert for traffic when entering or leaving the stops. Several stops are on private gravel roads whose owners have kindly allowed public access. Please respect these and other properties along the way. Do not walk on commercial blueberry fields or pick the berries.
Global changes during the Ice Age
Rapid and dramatic changes in global climate have occurred during the last 2.5 million years of the Earth's history, called the Ice Age. These changes caused large glaciers of continental proportions (ice sheets) to repeatedly expand and contract across northern regions of North America, Europe, and Asia. Global sea level dropped during each glacial episode, when great volumes of water were tied up in the ice sheets. At the same time the weight of the thick continental ice sheets was so enormous that it depressed the underlying Earth's crust, which later rose back to its former position when the ice melted away. The interaction of these fluctuating land and sea levels resulted in alternate flooding and emergence of coastal areas including southern Maine.
Some areas of the world that are now desert were vegetated during the Ice Age and supported herds of animals. The cool moist climate produced large lakes that have since dried up or greatly diminished. Giant Arctic fauna including mammoths, rhinoceros, reindeer, musk ox, and bison lived on the tundra and grassland which covered most of Europe and parts of North America south of the ice sheets. And finally, the last 2.5 million years saw the evolution of modern humans. Archaeologists have found evidence of Paleoindian settlement in New England immediately following the retreat of the last ice sheet.
The fossil record
There are two general methods for determining when events happened in Maine during the Ice Age. One of these is "relative dating," in which one feature is determined to be younger or older than another. This method can be applied, for example, in places where sediment layers are stacked in a sequence. The older layers are toward the bottom and the younger layers on top. No actual ages can be assigned by this method. The other technique - absolute dating - determines specific ages in years. In the late 1940's scientists developed the method called radiocarbon dating, which is useful for accurate dating of organic remains back to about 40,000 years ago. This method is based on the fact that plants and animals incorporate a radioactive carbon isotope (carbon-14, represented by the symbol 14C) from the atmosphere. Following the death of an organism, the percentage of 14C that it contains becomes less with time. The ratio of remaining 14C to stable carbon (12C) can be measured in a laboratory and used to determine the age of fossils such as shells, wood, and seaweed. A brief introduction to radiocarbon dating is available at the University of Oxford radiocarbon web-info website.
Numerous 14C ages have been obtained from fossils found in Maine and neighboring areas. Many of them came from shells such as those of clams and mussels, which were collected from the marine clays. Other ages were determined from terrestrial plant matter preserved in lake bottoms and peat bogs. It has been found that 14C ages from late-glacial time typically are at least 2000 years younger that the true ages of the fossils in actual calendar years. An additional correction should be applied to marine fossils, whose measured radiocarbon ages are several hundred years too old relative to terrestrial fossils. Unless otherwise noted, we will express ages here in calendar years before present.
The Ice Age in Maine
As in other parts of the world, each successive advance of the continental ice sheets modified or obliterated the record of earlier glaciations. In many places older glacial features were either buried by the sedimentary deposits of younger ice sheets, or they were more-or-less eroded away by the flowing ice. The oldest dated glacial sediments in Maine occur in New Sharon. Associated fossil plant remains yielded an age greater than 52,000 years. The most recent advance of ice, the Laurentide Ice Sheet, expanded across the St. Lawrence Lowland in Quebec about 35,000 years ago. The ice flowed southeastward across New England, terminating at Long Island and along Georges Bank in the Gulf of Maine between about 28,000 and 24,000 years ago. During this time of maximum glacial expansion, the ice sheet covered all of Maine to a depth of greater than 2.4 km (1.5 mi). The heavy load of the glacier pushed the Earth's crust downward over 150 m (500 ft).
With rapidly rising global temperatures, the southern margin of the Laurentide Ice Sheet began to retreat from coastal southern New England and the Gulf of Maine shortly before 21,000 years ago. Global sea level had been lower than present during the glacial maximum, but the land remained depressed for a time as the ice sheet retreated. This lingering depression enabled the sea to flood low areas in southern Maine as the ice withdrew. Around 16,000 to 15,000 years ago the ice margin was retreating inland in shallow marine waters. The glacial landforms associated with this marine submergence are among the most distinctive features of the Ice Age Trail.
Just after glacial retreat, much of Maine was a treeless tundra that supported large animals such as wooly mammoths. Warming of the climate allowed the northward migration of a cold boreal forest, which in turn was replaced by the forest of today. The first humans, the nomadic Paleoindians, arrived in Maine between about 13,000 and 11,500 years ago during the extremely cold time called the Younger Dryas interval, while residual masses of glacial ice still existed in parts of northern Maine.
"Ground moraine" is sediment deposited directly from a glacier and usually composed of mixed rock debris ranging from clay to boulder size. This material, called till, forms a stony blanket over much of Maine. Till is most noticeable on hills where it isn't covered by younger sediments. An "end moraine" (henceforth simply called moraine) is a ridge of sediment deposited along the margin of a glacier. It can vary widely from till laid down at the base of the glacier to stratified sand and gravel deposited by meltwater flowing out of the ice. The ridge shape of a moraine may result from bulldozing by advancing ice, or from a situation in which internal flow of ice is balanced by melting at its margin. This allows the ice margin to remain in a stable position for a while, during which sediments are conveyed forward and collect in a ridge along the front of the glacier (Figure 5).
Moraines are abundant in many areas along the Ice Age Trail (Figure 6 and Figure 7), where they mark successive positions of the retreating glacier margin. In some cases the close spacing of moraines may have resulted from annual deposition. The ridges are best seen in the open blueberry fields, and they are strewn with large boulders dumped off the glacier. The larger moraines are hundreds of feet across and thousands of feet long. The best known example is the Pineo Ridge Moraine, which is actually a belt of large moraines extending from northeast of Cherryfield east to Lubec (Stops 28 and 39 on the Ice Age Trail). A few end moraines such as Pond Ridge in Cutler (Stop 34) contain marine clay layers with organic remains whose ages have been determined by radiocarbon dating.
Fans and deltas
Wherever a stream flows into a lake or ocean, its velocity and sediment transporting power quickly diminish. Any sand and gravel that it carried will accumulate at the mouth of the stream. In coastal Maine the margin of the Laurentide Ice Sheet stood in the sea, so the coarse sediments were dumped into the ocean where meltwater poured out from the mouths of ice tunnels. The resulting mounds of sand and gravel are called "submarine fans" if they did not build up to the ocean surface (e.g. Stop 27). Other deposits grew until they reached sea level and developed flat tops as glacial stream action smoothed their upper surfaces. The latter are known as "deltas" (Figure 8 and Figure 9). An ideal delta is triangular in shape with its apex pointed upstream, like the famous delta at the mouth of the Nile River in Egypt. Good examples of deltas can be seen in several places along the Ice Age Trail, including Stops 7, 12, and 19.
Much of the sandy sediment in a glacial-marine delta cascaded down the front, forming a series of sloping layers (foreset beds) that built the delta out into the ocean. As the delta expanded seaward, the surface streams extended their channels to the front edge and deposited a horizontal gravelly layer (topset beds) above the foresets (Figure 10). The boundary between topsets and foresets records the position of sea level when the delta formed, while the thickness of the topset beds reflects the tidal range. In eastern coastal Maine there are many prominent deltas that formed at the glacier margin. They are concentrated at the points where prodigious amounts of meltwater and sediment discharged from subglacial tunnels. When retreat of the glacier margin removed the support from the back side of each delta, the sand and gravel often collapsed, yielding hummocky "kame-and-kettle" topography.
Some of the largest deltas in eastern Maine were constructed along the trend of the Pineo Ridge Moraine when the glacier margin stood still or readvanced slightly for a considerable time. The sea-level marked by these deltas was approximately 76 m (250 ft) higher than present and the tidal range was about 3 m (10 ft) compared to as much as 9 m (30 ft) today.
Raised beaches and marine muds
During and following glacial retreat, the land rose more rapidly than global sea level and the deltas emerged. Occasionally the land and sea rose at the same rate, during which the position of the ocean shoreline remained constant and wave action carved beach terraces on the delta fronts. These beaches are now high and dry following uplift of the land (Stop 26) (Figure 11). Lower areas that were drowned by the sea are covered with mud deposited in quiet water on the ocean floor (Figure 12). These clay-rich sediments are known as the Presumpscot Formation, after the Presumpscot River valley in Portland, Maine. In some places they contain fossils including species that are typical of sub-arctic environments (Stop 15).
Non-technical web sites and published maps
Kelley, J. T., Dickson, S. M., and Belknap, D. F., 1998, Maine's History of Sea-Level Changes, Maine Geological Survey fact sheet.
Thompson, W. B., 2000, Maine's Glacial Moraines: Living on the Edge, Maine Geological Survey fact sheet.
Thompson, W. B., 2003, Maine's Glacial Deltas, Maine Geological Survey fact sheet.
Thompson, W. B., and Borns, H. W., Jr., 1985, Surficial Geologic Map of Maine: Maine Geological Survey, 1:500,000-scale map. Small-scale version with supplemental information available in pdf format (2 Kb).
Weddle, T. K., 2000, A General Introduction to the Presumpscot Formation - Maine's "Blue Clay": Maine Geological Survey fact sheet.
Links to press releases on Maine's Ice Age Trail
Scientific literature, including more detailed information on the glacial geology of eastern Maine
Ashley, G. M., Boothroyd, J. C., and Borns, H. W., Jr., 1991, Sedimentology of late Pleistocene (Laurentide) deglacial-phase deposits, eastern Maine; An example of a temperate marine grounded ice-sheet margin, in Anderson, J. B., and Ashley, G. M. (editors), Glacial marine sedimentation: paleoclimatic significance: Geological Society of America, Special Paper 261, p. 107-125.
Borns, H. W., Jr., and 8 others, 2004, The deglaciation of Maine, U.S.A., in Ehlers, J., and Gibbard, P. L. (editors), Quaternary glaciations - extent and chronology, Part II (North America): Elsevier, p. 89-109.
Dickson, S. M., 1999, The role of storm-generated combined flows in shoreface and inner continental shelf sediment erosion, transport, and deposition: Ph.D. dissertation, University of Maine, Orono, 321 p.
Ridge, J. C., 2004, The Quaternary glaciation of western New England with correlations to surrounding areas, in Ehlers, J., and Gibbard, P. L. (editors), Quaternary glaciations - extent and chronology, Part II (North America): Elsevier, p. 169-199.
Stuiver, M., and Borns, H. W., Jr., 1975, Late Quaternary marine invasion in Maine: Its chronology and associated crustal movement: Geological Society of America, Bulletin, v. 86, p. 99-104.
Thompson, W. B., 1982, Recession of the Late Wisconsinan ice sheet in coastal Maine, in Larson, G. J., and Stone, B. D. (editors), Late Wisconsinan glaciation of New England: Kendall/Hunt, Dubuque, p. 211-228.
Text by Woodrow B. Thompson (Maine Geological Survey) and Harold W. Borns, Jr. (University of Maine - Climate Change Institute).
Originally published on the web as the July 2007 Site of the Month.
Last updated on January 20, 2011
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