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Evidence for a calving embayment in the Penobscot River Valley, Bangor, Maine
During the summer of 2007, Dr. Kent M. Syverson and field assistant Andrew H. Thompson from the University of Wisconsin - Eau Claire were contracted by the Maine Geological Survey (MGS) to map the surficial geology of the Bangor 7.5-minute topographic quadrangle as a part of the STATEMAP component of the National Cooperative Geologic Mapping Program. The following Geologic Site of the Month is prepared from their map and report, cited below.
Evidence for Penobscot River valley calving embayment
Glacial calving, the spalling of ice from the front of the glacier, removes ice mass rapidly in deep water (Benn and Evans, 1998, p. 277-278). Based on the topset-foreset contact in the Hampden delta, a glaciomarine delta 9 mi (13 km) southwest of the Bangor city center, late Wisconsinan marine water depths ranged from 0 to 317 ft (97 m) in the Penobscot River lowland near Bangor (delta #63, Thompson and others, 1989). However, areas to the west, south, and east rose well above the marine limit. Some workers informally have suggested that the ~100 m difference in sea-water depth caused enhanced calving and a calving embayment in the Penobscot River valley -- a controversial idea.
According to Lowell (1994), an embayment did not form in the Penobscot River lowland because the deep-water area was too narrow. The goal of this part of the study was to examine ice-flow indicators near Bangor and determine if a calving embayment formed in the Penobscot River lowland. If a calving embayment once existed, then ice-flow directions should have changed markedly and converged along the deepest part of the Penobscot River valley during deglaciation.
Erosional features were measured at 51 individual sites across the entire quadrangle (Figure 4; Thompson and Syverson, 2008). These included striations (non-unique flow indicators) and crag-and-tail features (unique flow indicators) (Figure 5). The azimuth and average size of each striation set were recorded, and any cross-cutting relationships and/or unidirectional crag-and-tail features were noted. We assigned relative ages to multiple striation sets whenever possible using the relative-size criterion where deep striations represent older flow events and more subtle sets represent younger overprinting (Syverson, 1995).
Once all field data was collected, flow-indicator data was entered in Microsoft Excel. Azimuth, location (either west or east of Penobscot River), uniqueness, and relative age for each erosional indicator set were coded and then sorted by location. ROCKWORKS 99 software was used to plot rose diagrams and analyze flow trends. Vector means were calculated for the orientations of unique flow indicators (crag-and-tail features). Maps, the relative ages of cross-cutting striation sets, and rose diagrams were evaluated to determine ice-flow direction changes.
A continuous range of abrasion marks is observed west of the Penobscot River. Outcrops at Brown Woods and the Interstate Highway 95/Stillwater Avenue intersection show the relationships most clearly (Figure 6). Robust striation and crag-and-tail features from the flow maximum (175° azimuth) are cut by less well developed striation and crag-and-tail features oriented in a more easterly direction (between 175° and 100° azimuths). Thus, flow became more easterly toward the Penobscot River valley as deglaciation proceeded. These changes in flow direction were observed up to 3 km west of the Penobscot River (Lane Corporation rock quarry on Odlin Road).
Ice-flow directions changed markedly in the Bangor area during deglaciation. Flow patterns west of the Penobscot River changed from southerly (175° azimuth vector mean) to easterly (100° azimuth) as deglaciation proceeded - a 75° flow change (Figure 6). The continuous range of striations and crag-and-tail features suggests a progressive easterly shift in the ice-flow direction. The change in flow direction is even more pronounced east of the Penobscot River - a 105° change from southerly (175° azimuth vector mean) to westerly (280° azimuth vector mean, Figure 7). Intermediate values between these two sets are lacking in this area suggesting a rapid change in flow direction from southerly to the west-northwest.
Although the convergent ice-flow pattern is distinctive along the Penobscot River, other non-calving explanations must be considered. First, the changing flow patterns could have occurred during a separate readvance of the ice margin. This seems unlikely from a glaciological standpoint, and evidence for such an event is lacking in the literature (Thompson and Borns, 1985; Borns and others, 2004). Secondly, ice flow toward the Penobscot River valley could have occurred as ice slid directly down the bedrock slope during the latest stages of deglaciation, as observed by Mickelson (1971). This seems unlikely in the Bangor region because (1) the Penobscot valley walls generally rise ~90 ft (27 m) within ~1600 ft (0.5 km) of the river up to a rather gently sloping bedrock platform; secondary flow indicators from this study are found on the gently sloping bedrock platform more than ~1600 ft (0.5 km) from the steep valley walls, (2) the abrasion marks associated with the secondary flow directions are extremely robust in many places, especially at the Interstate 395/Parkway South intersection in Brewer, and (3) the secondary striation patterns are very consistent; variations in bedrock slope direction would produce disparate striation orientations.
We conclude that a narrow calving embayment is the most reasonable explanation for convergent ice-flow patterns along the Penobscot River lowland near Bangor (Figure 8). The Penobscot valley is quite narrow, and water depths would have varied by ~98 ft (30 m) within a distance of 3280 ft (1 km) on either side of the river. Based on this, we think the calving embayment was <1.2 mi (2 km) wide when it formed. Impacts of this calving embayment have been observed up to 1.8 mi (3 km) west and 0.9 mi (1.5 km) east of the Penobscot River.
Benn, D.I. and Evans, D.J.A., 1998, Glaciers & Glaciation: New York, John Wiley & Sons, Inc., 734 p.
Borns, H.W., Jr., Doner, L.A., Dorion, C.C., Jacobson, G.L., Jr., Kaplan, M.R., Kreutz, K.J., Lowell, T.V., Thompson, W.B., and Weddle, T.K., 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 Publishing, Amsterdam, p. 89-109.
Lowell, T.V., 1994, Maine's calving bay?: Geological Society of America, Abstracts with Programs, v. 26, no. 3, p. 57.
Mickelson, D.M., 1971, Glacial geology of the Burroughs Glacier area, southeastern Alaska: Ohio State University Institute of Polar Studies, Report 40, 149 p.
Ridge, J.C., 2008, "The North American Glacial Varve Project": sponsored by The National Science Foundation and The Geology Department of Tufts University, Medford, Massachusetts.
Ridge, J.C., Canwell, B.A., Kelly, M.A., and Kelley, S.Z., 2001, An atmospheric 14C chronology for Late Wisconsinan deglaciation and sea level change in eastern New England using varve and paleomagnetic records, in Weddle, T., and Retelle, M. (editors), Deglacial history and relative sea-level changes, northern New England and adjacent Canada: Geological Society of America, Special Paper 351, p. 171-189.
Strömberg, B., 1981, Calving bays, striae and moraines at Gysinge-Hedesunda, central Sweden: Geografiska Annaler, v. 63A, p. 149-154.
Syverson, K.M., 1995, The ability of ice-flow indicators to record complex, historic deglaciation events, Burroughs Glacier, Alaska: Boreas, v. 24, p. 232-244.
Syverson, K.M., and Thompson, A.H., 2008, Surficial geology of the Bangor 7.5-minute quadrangle, Maine: Augusta, Maine Geological Survey, Open-File Map 08-37, scale 1:24,000.
Thompson, A.H. and Syverson, K.M., 2008, Evidence for a calving embayment in the Penobscot River valley, Bangor, Maine: Geological Society of America, Abstracts with Programs, v. 40, no. 5, p. 71-72.
Thompson, W.B., 2007a, Surficial geology of the Augusta 7.5' quadrangle, Maine: Augusta, Maine Geological Survey, Open-File Map 07-84, scale 1:24,000.
Thompson, W.B., 2007b, Surficial geology of the Gardiner 7.5' quadrangle, Maine: Augusta, Maine Geological Survey, Open-File Map 07-103, scale 1:24,000.
Thompson, W. B., and Borns, H. W., Jr. (editors), 1985, Surficial geologic map of Maine: Maine Geological Survey, scale 1:500,000.
Thompson, W.B., Crossen, K.J., Borns, H.W., Jr., and Anderson, 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., eds., Neotectonics of Maine: Augusta, Maine Geological Survey Bulletin 40, p. 43-67.
Thompson, W.B., Griggs, C.B., Miller, N.G., and Weddle, T.K., 2008, Associated terrestrial and marine fossils in the late-glacial Presumpscot Formation, southern coastal Maine, and the marine reservoir effect on radiocarbon ages: Geological Society of America, Abstracts with Programs, v. 40, no. 2, p, 51.
Text and photos by Kent Syverson and Andrew Thompson; compilation and editing of website by Thomas Weddle.
Originally published on the web as the December 2008 Site of the Month.
Last updated on April 23, 2012
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