Mason County, Illinois  Mason County is a county in the U.S. state of Illinois. According to the 2010 census, it had a population of 14,666. Its county seat is Havana. The county is named in honor of George Mason, a member of the Virginia legislature who campaigned for the adoption of the United States Bill of Rights. Mason County was created in 1841 out of portions of Tazewell and Menard counties. According to the U.S. Census Bureau, the county has an area of , of which is land and (4.3%) is water. Mason County is bound on the south by the Sangamon River, and on the west by the Illinois River. These rivers join at the county's southwest tip. The soil covering much of Mason County is very sandy. This was formed during the melting of the Wisconsin Glacier about 10,000 years ago. Meltwater from the glacier deposited large amounts of sand in a delta region near at the junction of the Sangamon and Illinois Rivers. The sandy soil does not hold water well, quickly exposing crops to drought conditions as the water table drops during periods of low precipitation. However, the soil is very good for growing vegetables that are otherwise not common in Illinois. Modern irrigation has made this a highly productive agricultural area. A sand wetland on the Illinois River is managed by the U.S. Fish and Wildlife Service as the Chautauqua National Wildlife Refuge. In recent years, average temperatures in the county seat of Havana have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1999 and a record high of was recorded in July 1983. Average monthly precipitation ranged from in January to in May. As of the 2010 United States Census, there were 14,666 people, 6,079 households, and 4,060 families living in the county. The population density was . There were 7,077 housing units at an average density of . The racial makeup of the county was 98.1% white, 0.4% black or African American, 0.3% Asian, 0.2% American Indian, 0.1% from other races, and 0.9% from two or more races. Those of Hispanic or Latino origin made up 0.8% of the population. In terms of ancestry, 33.8% were German, 15.6% were American, 11.1% were English, and 10.3% were Irish.