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Big Rock - An Erratic in Waltham, Maine
This Geologic Site of the Month is another locally famous, very large boulder (see the websites about Balancing Rock and Daggett Rock) that was transported to its location by continental glacier during the Ice Age. Such boulders are often called erratics because they are composed of rock that is generally (but not necessarily) different from the bedrock beneath the boulder.
Directions: From Ellsworth head north on Route 179 to Cave Hill Road (Route 200) and turn right (East). Continue until you see Leona Wilbur Road and turn right (South) and follow road to intersection with road where a sign for Big Rock is seen; turn right and proceed to end of road and park. Follow path to Big Rock.
The boulder: Big Rock's dimensions are approximately 30 feet wide by 30 feet high by 60 feet in length. It is approximately 54,000 cubic feet and weighs about 3375 tons.
Views of Big Rock
The granite: The size of the boulder itself is very impressive, but Big Rock also has features in it that tell us something about its origin, long before it became a glacial erratic boulder.
Closeups of granite and interesting details
Source area: Before it was carried to this place by the glacier, Big Rock was originally part of the bedrock. The granite of Big Rock can be compared to bedrock in the area to try to determine where the rock may have come from. The Geologic Map (of Osberg and others, 1985) shows that the bedrock beneath Big Rock is the Deblois Granite. There is a bedrock exposure of Deblois Granite in a small gravel pit near the Big Rock parking area. Another possibility is that Big Rock could have come from the Lucerne Granite, which is found to the northwest of Big Rock (see Geologic Map). Unfortunately, the Lucerne and Deblois granites can be quite similar in mineral composition and texture, and in the size of their mineral grains. So if Big Rock is from the Deblois, it could have been transported a very short distance, and if it is from the Lucerne, it may have been transported as far as ten miles by glacier ice.
Visit Big Rock for yourself and see what you can find.
Osberg, Philip H., Hussey, Arthur M., II, and Boone, Gary M. (editors), 1985, Bedrock geologic map of Maine: Maine Geological Survey (Department of Conservation), scale 1:500,000.
Text and photos by Thomas K. Weddle unless otherwise noted.
Originally published on the web as the November 2012 Site of the Month.
Last updated on December 13, 2012
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