There are 9 US Post Offices in Grundy County serving a total of 51,030 residents equating to 5,670 residents per post office. It's estimated that approximately 81,450 packages are handled across the 9 county post offices annually. Grundy County is a county located in the U.S. state of Illinois. According to the 2010 census, it has a population of 50,063. Its county seat is Morris. Grundy County is part of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area. In 2010, the center of population of Illinois was located in Grundy County, just northeast of the village of Mazon. Illinois's State Fossil, the unique and bizarre Tully Monster, was first found in Mazon Creek. Grundy County Speedway is located in Morris. Grundy County is also home to Dresden Generating Station\u2014the first privately financed nuclear power plant built in the United States\u2014and the Morris Operation\u2014the only de facto high-level radioactive waste storage site in the United States."