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Tracy Corners Gravel Pit, Addison
The gravel pit exposure at Tracy Corners in southeastern Maine, approximately 1.8 miles south of the junction U.S. Route 1 and State Route 187 in Addison, is a well known field trip stop for students of glacial geology. It has been the subject of detailed geological study (Ashley, Boothroyd, and Borns, 1991), and is featured in the cover photograph of the Studies in Maine Geology, Volume 6: Quaternary Geology published by the Maine Geological Survey. It can be located on the Columbia Falls 7.5-minute USGS topographic quadrangle.
The pit is excavated into a landform known as a moraine. A moraine is a ridge of sediment formed at the front of a glacier, and can be formed by deposition of sediment from the melting ice margin, or by pushing of the sediment by the glacier as it moves forward. In this exposure there is evidence for both types of moraine-forming activity.
The landform in Addison is part of a greater moraine complex in eastern Maine, the larger components of which are termed the Pond Ridge moraine (east of Addison) and the Pineo Ridge moraine system (north of Addison) (Kaplan, 1994). Both of these larger moraine systems, as well as the smaller moraines like the one at Addison formed as the ice margin retreated in contact with the ocean between about 16,800 and 16,000 calendar years ago. The ice margin was grounded in the sea, and each moraine represents a "grounding-line position," that is, a place where the ice front stood long enough for a moraine to form. The moraines have cross-cutting and parallel orientations with respect to each other. This change in sediment deposition suggests that the ice margin was moving, to readvance or overlap its former position. On a regional scale in southeastern Maine, these landforms clearly reflect the shape of the ice margin at that time as a lobate form with embayments in the lowlands.
Along coastal Maine, radiocarbon ages on marine fossil shells imply that ice in coastal eastern Maine was separated from ice in coastal western Maine by a major marine embayment in the Penobscot River valley which was extant as far inland as Medway at about the time the moraine at Addison formed. During this phase of deglaciation, did the eastern Maine ice behave as a completely separate ice mass from the ice in western 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: Boulder, Colorado, Geological Society of America, Special Paper 261, p. 107-125.
Kaplan, M.R., 1994, The deglaciation of southeastern Maine: M.S. thesis, University of Maine, Orono, 112 p.
Originally published on the web as the February 1998 Site of the Month.
Web site by Thomas K. Weddle
Last updated on June 6, 2010
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