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by Nora M. |
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Madison Boulder is a huge piece of granite that was dragged by an ice sheet called a glacier. It is the largest known glacial erratic in North America and has been recognized by the U.S. Department of the Interior as a National Natural Landmark. A glacial erratic means that a boulder has been moved from its original place by the ice sheet to a new place. The most recent ice age, the Wisconsin Glacial Stage, occurred between 12,000 and 16,000 years ago. The continental ice mass pushed animal and plant species south as it moved down from Greenland, at the rate of 6-8 inches per day. In some places, the mass could speed up to 12-18 inches per day. Over time, the glacier built up to a mile in thickness. Markings on Mount Washington's summit show that the ice mass covered even that peak. As the glacier dragged many things through the White Mountain region, the pressure and weight of the ice caused pieces of exposed ledges of the hard granite of the area to break off from time. The largest rocks carried by ice sheets are called glacial erratics. Madison Boulder was "plucked from a ridge to the Northeast in Albany", about 2 miles from where it stand today. Madison Boulder is 87 feet long, 23 feet wide, and 37 feet high, and an estimated 4,662 tons. It is a mix of biolite and quartz feldspar known as Conway granite.
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