Marshall Shore Town Park, Liberty, Waldo County

At the southwest end of Lake St. George, the town of Liberty maintains a small picnic and swimming area. Although there are many camps at this end of the lake, this particular piece of property has bedrock very near the surface so it would be a difficult place for building. A public park is an excellent use for it. (See Location Map.)

Lake St. George Granite Gneiss

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Smooth, bare bedrock ledge is exposed near the swimming area. This is a good place to crawl around and study fascinating features of the rock. It is bleached nearly white on weathered surfaces, though broken pieces show that the fresh rock has a subtle light bluish-gray hue. The most distinctive feature of the rock is a streaked texture that looks something like wood grain. This feature, called foliation (Photo 3) is produced by heat and pressure which cause the minerals of the rock to become rotated and distorted into parallel alignment. Foliation in rocks is generated at depth in the earth, over geologic time spans. The process of transforming a solid rock into a foliated rock is called metamorphism. The rock at this site is a metamorphic rock called gneiss.

Map 2

For the most part, the gneiss has the same mineral composition as granite, namely quartz, feldspar, and black mica (biotite). Furthermore, microscopic study has shown that despite showing ragged and broken edges and other effects of metamorphism, some of the individual feldspar mineral grains retain characteristics typical of feldspar found in ordinary granite. Therefore, this gneiss is interpreted to have been a granite originally, before it was converted to gneiss through the process of metamorphism. The ledges at the swimming area are part of an extensive body of this kind of rock, first mapped by Kost Pankiwskyj (1976), and named the Lake St. George Granite Gneiss by Tucker and others (2001). Its northeast end is here in the lake, and it has been traced about 10 miles to the southwest (See Geologic Map). A sample of rock was taken from the blasted area across the road and analyzed in the laboratory. It gave an age of 422 million years old (with an analytical uncertainty of 2 million years), which is at the end of the Silurian Period of geologic time. This is the age of the original granite; the metamorphism must have occurred sometime later.

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.

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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.

remains of thin pegmatite layer in gneiss

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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.

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References

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.

Osberg, Philip H., Hussey, Arthur M., II, and Boone, Gary M. (editors), 1985, Bedrock geologic map of Maine: Maine Geological Survey, scale 1:500,000.

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