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is a suburban town in Middlesex County, Massachusetts, United States about twenty-one miles west-northwest of Boston along Route 2 west of Concord and about ten miles (18 km) southwest of Lowell. The population was 20,331 at the 2000 census. It is bordered by Westford and Littleton to the north, Concord and Carlisle to the east, Stow and Maynard to the south, and Boxborough to the west. Acton became an incorporated town in 1735. The town is run by five selectmen and a town manager.
Acton is located at . According to the United States Census Bureau, the town has a total area of 20.3 square miles (52.5 km²), of which 20.0 square miles (51.7 km²) is land and 0.3 square miles (0.8 km²), or 1.53 percent, is water. Most of the land may be described as rocky hills. Almost all of Acton is forested, except for where it has been cleared for residential or agricultural use.
The current geography of Acton was created when the last wave of glaciers retreated approximately ten thousand years ago. Acton has nine drumlins — hills which are composed of glacial till. In addition, Wills Hole and Grassy Pond are kettle ponds which were formed in depressions in the till formed by large blocks of ice.
Acton has two primary stream systems: the Nashoba Brook system including the incoming streams Butter Brook, Will's Hole Brook and Conant Brook and the Fort Pond Brook system including the incoming streams Guggins Brook, Inch Brook, Grassy Pond Brook, Pratt's Brook and Coles Brook. Both stream systems empty into the Assabet River. Acton borders on Nagog Pond in the north, and there is a small artificial pond at Nara Park in North Acton.