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Marshall Shore Town Park, Liberty, Waldo County
Lake St. George Granite Gneiss
White streaks through the gneiss are remnants of dikes or veins in the original granite that were squeezed, stretched, and twisted during metamorphism. The shapes of these lighter colored streaks and stripes give a clue to the amount of distortion this rock has been through. In some places, even the foliation is curved or is cut off against other foliation to produce complicated designs.
Pegmatite Intrusion and Deformation
Near the upper part of the rock exposure, there is a prominent white layer a foot or two thick, running through the gneiss. The mineral grains in this layer are very large compared to those of the ordinary gneiss. This layer was derived from a coarse-grained variety of granite called pegmatite. The pegmatite intruded the granite before metamorphism, as can be seen from the fact that it is broken and deformed. While the middle of the pegmatite is fairly well preserved, the edge of the pegmatite has been very strongly affected by metamorphism. In places, the margin of the pegmatite has been completely disassembled, and individual large feldspar grains are embedded in the gneiss.
Elsewhere, strings of a few large, harried feldspar grains are all that remain of former thin pegmatite layers. Even isolated single feldspar grains in the gneiss beg the question as to whether they were derived by extreme dismemberment of pegmatite that has been mechanically mixed into the gneiss. By carefully trying to trace these layers through the gneiss, it becomes apparent that this rock has been thoroughly distorted and distended by the metamorphic process.
Surface Marks left by the Continental Glacier
Around the corner from the swimming area, in front of the picnic tables, more flat bedrock surfaces extend into the lake. In a few places, especially back from the water where the rock has been protected by soil and vegetation, there are faint scrape marks on the bedrock surface. The marks are approximately parallel to each other and trend toward the south. They were made by stones embedded in the base of the continental ice sheet that moved across Maine during the last Ice Age. These marks are about 14,000 years old, and indicate that the bedrock surface we see today is a natural surface that is essentially unchanged since that time. Striations and grooves like this can be found on bedrock surfaces across New England. They show the direction the ice was moving, in this case almost due South (177 to 179 degrees). If you visit here, please do not damage the natural marks! After 14,000 years, let's hope they last a hundred more.
Pankiwskyj, Kost A., 1976, Preliminary report on the geology of the Liberty 15' quadrangle and adjoining parts of the Burnham, Brooks, Belfast, and Vassalboro (15-minute) quadrangles in south-central Maine: Maine Geological Survey, Open-File Report 76-29, 8 p. (map, scale 1:62,500).
Tucker, Robert D., Osberg, Philip H., and Berry, Henry N., IV, 2001, The geology of a part of Acadia and the nature of the Acadian Orogeny across central and eastern Maine: American Journal of Science, v. 301, no. 3, p. 205-260.
Photos and text by Henry N. Berry IV.
Originally published on the web as the August 2005 Site of the Month.
Last updated on October 6, 2005
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