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Maine's Glacial Deltas
(It's a delta for shore!)
How deltas formed
Large areas in southern Maine were underwater during the retreat of the most recent glacial ice sheet. The earth's crust had been pushed down hundreds of feet by the weight of the glacier, and this depression persisted long enough for the ocean to flood valleys and other low areas of southern Maine as soon as the glacier departed. In the Kennebec and Penobscot valleys, the sea extended all the way to Bingham and Millinocket in central Maine.
Vast quantities of sediment were dumped into the ocean at the edge of the melting ice sheet, and these deposits are now exposed to view because the land rose above sea level soon after glacial retreat. The fine-grained muds dispersed onto the ocean floor, forming a blanket of clay-rich sediment called the Presumpscot Formation. However, the coarser sediments were dropped right at or near the edge of the glacier. These ice-marginal deposits include moraine ridges, which often formed in deeper waters and may include both glacial till and sand/gravel (Figure 1). Sand and gravel also discharged in large quantities from glacial tunnels, building submarine fans. Wherever the sediment supply was adequate and the glacier margin remained in one place long enough, these sediments built up to the ocean surface, eventually becoming flat-topped deposits known as glaciomarine deltas, or simply "marine deltas" in the discussion below. Figure 2 shows an example of one of these deltas.
Some of the marine deltas are very large, with surface areas measurable in square miles. A prominent example in southwestern Maine is the cluster of deltas in Gray and New Gloucester, crossed by the Maine Turnpike. The Augusta airport, North Windham village, and Sebago Lake village are likewise situated on the flat tops of marine deltas. Thompson and others (1989) located and described over 100 marine deltas in southern Maine, and there are many others which have not been studied in detail. These deltas are especially large and numerous along the inland limit of marine submergence, with tops commonly at elevations of 200-400 feet above present sea level (highest to the northwest).
Farther inland, above the reach of the sea, glaciolacustrine deltas were deposited into short-lived glacial lakes. Many of these lakes formed in valleys where meltwater draining from the glacier was dammed by the ice itself, especially in valleys that sloped north toward the glacier. Lakes also existed where plugs of glacial sediment temporarily blocked the south-draining streams. Some of the glacial lakes emptied and disappeared as soon as the glacier retreated or erosion breached the dams, while others were totally filled with deltaic sand and gravel.
Characteristics of deltas
Geologists have learned much about deltas from their surface topography and especially from sand and gravel pits that expose their interior structure. Maine's glacial deltas - both marine and lacustrine - are a distinct type of deposit called the "Gilbert delta," named after G. K. Gilbert of the U.S. Geological Survey. In the late 1800's Gilbert pioneered the description of coarse-grained deltas, which differ from the broad gently-sloping muddy deltas such as that of the Mississippi River. The interior of a Gilbert delta typically has three stratigraphic components. From top to bottom, these are the topset, foreset, and bottomset beds (Figure 4). The topset beds form a horizontal layer extending across the top of the delta. They were deposited by streams flowing across the delta surface and are usually coarser than the rest of the delta. The "topsets" are rich in gravel, sometimes including boulders to 3 feet or larger in locations close to the former ice margin (Figure 5). Most of these stream gravels are poorly sorted, but in some places there are channel structures indicating the direction of former stream flow (Figure 6).
Sediments that were carried all the way to the front of the delta cascaded down into deeper waters, forming the sloping foreset beds on the delta front. The foresets are inclined in the general direction of sediment transport and delta growth, and are typically composed of sand or mixed sand and gravel (Figures 7 and 8). As the delta built outward into the lake or ocean, the topset unit expanded such that the topset beds extended out over previously-deposited foresets. These sediments generally become finer grained with increasing distance of transport, though ice-rafted boulders may be found among them (Figure 9). The finest delta sediment occurs in the bottomset beds, deposited as horizontal or gently sloping layers on the bottom of the water body in front of the delta. The result of this process is the vertical sequence of topset/foreset/bottomset beds that we ideally see in a good cross section of a delta, especially in gravel pits.
If the original delta top is preserved (i.e. has not been washed over by the sea or modified by human activities), we sometimes see old channels carved by glacial streams flowing across the delta plain. These meltwater channels are best seen in open fields (Figure 10). In places where large chunks of glacial ice were buried in the delta, the subsequent melting of those ice blocks produced depressions called kettles. The abundance, depth, and shape of kettles vary widely, and some of them extend below the water table and have ponds or bogs in the bottom. Kettles, knobby terrain, and large boulders are most likely to occur in the headward (ice-contact) portions of deltas (Figure 11). The fronts of marine deltas may show step-like terraces marking successive shorelines as relative sea level fell in late-glacial time (Figure 12).
Deltas as dipsticks
The contact between topsets and foresets is generally sharp and horizontal. This boundary is very important because it closely approximates the position of lake or sea level when the delta was deposited. Glacial-lake shorelines are poorly expressed in Maine, and while there are numerous features indicative of former sea level, many of these old beaches formed as the land rose and relative sea level was falling. However, the ice-contact deltas reveal the original water levels at the time of glacial retreat. Thus the topset/foreset contact is a geological dipstick for determining former water levels, especially in the case of marine deltas, and for predicting where other geological evidence of past shorelines may be found.
Economic importance of deltas
Deltas are very important to Maine citizens for several reasons:
-- Deltas are major sources of ground water (aquifers). Most deltas extend below the water table and thus the deeper part of the deposit is saturated with ground water. The delta itself may be a high-yield aquifer, and it may also be a source of recharge for water flowing into adjacent ponds and lakes. Several Maine bottling plants have tapped marine and lacustrine deltas for large supplies of clean drinking water.
-- Deltas provide educational information regarding Maine's
glacial and sea-level history. These considerations have already
been discussed above.
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.
Crossen, K. J., 1991, Structural control of deposition by Pleistocene tidewater glaciers, Gulf of Maine, in Anderson, J. B., and Ashley, G. M. (editors), Glacial marine sedimentation: paleoclimatic significance: Geological Society of America, Special Paper 261, p. 127-135.
Gilbert, G. K., 1885, The topographic features of lake shores: U.S. Geological Survey, Fifth Annual Report, p. 69-124.
Gilbert, G. K., 1890, Lake Bonneville: U.S. Geological Survey, Monograph 1, 438 p.
Koteff, C., Robinson, G. R., and Goldsmith, R., 1993, Delayed postglacial uplift and synglacial sea levels in coastal central New England: Quaternary Research, v. 40, p. 46-54.
Miller, S. B., 1986, History of the glacial landforms in the Deblois region, Maine: M.S. thesis, Univ. of Maine, Orono, 74 p.
Thompson, W. B., and Borns, H. W., Jr., 1985, Surficial geologic map of Maine: Maine Geological Survey, 1:500,000-scale map.
Thompson, W. B., Crossen, K. J., Borns, H. W., Jr., and Andersen, B.G., 1989, Glaciomarine deltas of Maine and their relation to late Pleistocene-Holocene crustal movements, in Anderson, W. A., and Borns, H. W., Jr. (editors), Neotectonics of Maine: Maine Geological Survey, Bulletin 40, p. 43-67.
Thompson, W. B., and Koteff, C., 2002, Glaciomarine deltas in southwestern Maine: Indicators of late-glacial relative sea level and isostatic uplift: Geological Society of America, Abstracts with Programs, v. 34, no. 1, p. A-26.
Text, photos, and diagrams by Woodrow B. Thompson except as noted.
Originally published on the web as the December 2003 Site of the Month.
Last updated on October 6, 2005
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