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Maine's Glacial Moraines: Living on the Edge
Toward the end of the "Ice Age," a glacier of vast proportions covered Maine. This was the Laurentide Ice Sheet, which advanced southward out of Canada about 25,000 years ago and remained here for nearly 15,000 years. The slowly flowing ice was thick enough to cover Maine's highest mountains. It swept away much of the evidence of earlier glaciations, eroding both the bedrock and previously existing sediment cover. Many glacial features that we see today were actually left behind during the final northward retreat of the ice sheet, when the pulverized rock debris was released from the melting ice.
Even as the ice margin withdrew, internal flow within the glacier continued to transport its sediment load southward toward the edge of the ice sheet. Through a variety of processes, this dirty material was either released directly from the ice, forming a stony deposit called "till," or washed out of the glacier in meltwater streams. The water-laid sediments were deposited as layered accumulations in river valleys, lake basins, and Maine's coastal lowland. They include the majority of our sand and gravel deposits, and the finer silts and clays commonly found near the coast.
The clearest markers of glacial retreat are ridges of sediment called "end moraines," which will be referred to here as simply "moraines" (Figure 1). These ridges were heaped up along the edge of the glacier during brief periods (as short as a single year or season) when the ice margin remained in a stationary position or readvanced slightly. Moraines are interesting to geologists because they indicate both the position and orientation of the glacier margin at a particular point in time (Figure 2). They can also tell us something about the activity of the ice sheet. It should be noted that there are other kinds of moraine ridges in Maine, such as the prominent "ribbed moraine" around the Millinocket Lake region, which may have formed under the glacier rather than at its edge. Much remains to be learned about these puzzling features, and they will not be discussed here.
From a practical standpoint, many moraines provide important supplies of sand and gravel or sandy till that are useful for construction purposes. Depending on composition, they may also constitute significant aquifers. In many coastal communities, sandy moraine ridges provide good elevated building sites and opportunities for domestic sewage disposal in areas otherwise underlain by poorly-drained clay or bedrock outcrops. The large moraine fields in eastern Maine have sandy soils which are extensively cultivated for blueberries (Figure 3).
Moraines vary greatly in size, composition, origin, and abundance across the glaciated portion of the northern United States. In the Northeast, Long Island (New York) is a giant moraine complex marking the southern limit of the Laurentide Ice Sheet. Similar moraines occur on Cape Cod and the coastal islands of southern New England, and can also be traced across the intervening ocean floor. The eastward continuation of the glacial limit is offshore from the present Maine coast, but on land we can see moraines formed during glacial retreat.
The Coastal Moraine Belt
There are hundreds of these moraines in southern Maine. They are often associated with deltas and other deposits of glacial sand and gravel that washed into the ocean. Thanks to emergence of the land from the sea, these deposits are now clearly visible, and are among the best and most easily accessible concentrations of glacial-marine deposits in the western hemisphere. The great extent of the moraine belt can be seen on the Surficial Geologic Map of Maine (Thompson and Borns, 1985).
Individual moraine ridges typically range in size from a few feet to over 50 feet high, and in length from a few hundred to thousands of feet. Many of them, such as the ones in Figure 1 and Figure 2, are strewn with large boulders that spilled off the ice margin. The moraines occur in clusters of parallel ridges which show the pattern of ice retreat over a broad area. The smaller moraines are variously known as minor, DeGeer, or washboard moraines, and commonly have a regular spacing of 150-200 feet between successive ridges, suggesting they formed at regular (perhaps annual) intervals (Thompson, 1982). In some places the moraines are more or less concealed by younger glacial-marine sand, silt, and clay deposits that drape over them. Topographic maps, aerial photographs, and digital elevation models can be helpful for spotting these ridges.
The sand and gravel deposits associated with moraines are an important economic resource. Borrow pits are often excavated in these deposits, giving geologists the opportunity to study them in detail. A typical pit exposure in the coastal moraines shows till and/or coarse gravel deposited right at the ice margin, overlain by finer-grained well-stratified sediments (submarine fans) ejected from ice tunnels when the glacier margin had retreated a short distance (Figure 5). Brief forward pulses of the glacier have locally shoved and deformed the morainal sediments, creating structures such as folds and faults or plastering till against their north sides. In some case, sediments in the moraines are interlayered with marine clays, proving that the moraines were deposited in the sea (Figure 6). Fossil mollusk shells and seaweed may be found in the associated marine clays, enabling the age of the moraines to be determined by radiocarbon dating (Stuiver and Borns, 1975; Weddle and others, 1993).
In the eastern part of coastal Maine, many of the moraines are very large. They can be hundreds of feet across and over a mile long. These large moraines are usually stratified, consisting largely of sand and gravel deposited as aligned series of submarine fans. At each successive position of the glacier margin there must have been numerous closely spaced ice tunnels discharging water and sediment into the sea. Figure 7 shows a cross section through one such moraine at the well-known Tracy Corner pit in Addison. This deposit is mostly sand and gravel (fan material) with scattered lenses of till. As seen in the photograph, the glacier readvanced from left to right, causing the layers in the fan to be doubled over in a large fold structure. At the same time, a stony gray till layer was deposited on the "upglacier" (left) side of the moraine.
Moraines in the Interior of Maine
Moraines are much less common inland from the zone of marine submergence (and across the rest of northern New England) though the reason for this discrepancy is not clear. One of the largest and best known examples is the Basin Ponds Moraine, which extends along the lower east side of Mt. Katahdin (Davis, 1989). This and other moraines in the interior of the state were mostly deposited by the Laurentide Ice Sheet, although a few in northernmost Maine were formed by north(!)-flowing ice when a late-glacial ice cap had been isolated over that part of the state.
The large Androscoggin Moraine system on the Maine-New Hampshire border was deposited by a tongue of the Laurentide Ice Sheet that flowed eastward from the White Mountain region and down the Androscoggin River valley. Although similar in many respects to moraines formed by alpine glaciers in areas like the Rocky Mountains, this moraine system was the product of the continental ice sheet. No true alpine moraines are known on Katahdin or other high mountains in Maine. They may have existed at one time, when the cirque basins on Katahdin and other high peaks were formed by alpine glaciers, but were destroyed by the last episode of continental glaciation.
Davis, P. T., 1989, Late Quaternary glacial history of Mt. Katahdin and the nunatak hypothesis, in Tucker, R. D., and Marvinney, R. G. (editors), Studies in Maine geology - Volume 6: Quaternary Geology: Maine Geological Survey, p. 119-134.
Eusden, J. D., Jr., 1980, Surficial geology and late Wisconsinan history of the Worthley Pond 7.5-minute quadrangle, Maine: unpublished senior thesis, Bates College, Lewiston, 53 p.
Smith, G. W., and Hunter, L. E., 1989, Late Wisconsinan deglaciation of coastal Maine, in Tucker, R. D., and Marvinney, R. G. (editors), Studies in Maine geology - Volume 6: Quaternary Geology: Maine Geological Survey, p. 13-32.
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.
Thompson, W. B., and Borns, H. W., Jr., 1985, Surficial geologic map of Maine: Maine Geological Survey, 1:500,000-scale map. Click to see a simplified surficial geologic map of Maine.
Thompson, W. B., and Fowler, B. K., 1989, Deglaciation of the upper Androscoggin River valley and northeastern White Mountains, Maine and New Hampshire, in Tucker, R. D., and Marvinney, R. G. (editors), Studies in Maine geology - Volume 6: Quaternary Geology: Maine Geological Survey, p. 71-88.
Weddle, T. K., Koteff, C., Thompson, W. B., Retelle, M. J., and Marvinney, C. L., 1993, The late-glacial marine invasion of coastal central New England (northeastern Massachusetts - southwestern Maine): Its ups and downs, Chapter I in Cheney, J. T., and Hepburn, J. C. (editors), Field trip guidebook for the northeastern United States: 1993 Boston GSA - Volume 1: Department of Geology and Geography, University of Massachusetts, Amherst, Massachusetts, Contribution No. 67, p. I-1 to I-31.
Text and photos by Woodrow Thompson except as noted.
Originally published on the web as the January 2000 Site of the Month.
Last updated on October 6, 2005
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