'Clark County, IL' definitions:

Clark -- U.S. County in Illinois

From: Gazetteer 2000
Name :
Clark -- U.S. County in Illinois
Population (2000) :
17008
Housing Units (2000) :
7816
Land area (2000) :
501.503456 sq. miles (1298.887932 sq. km)
Water area (2000) :
3.379496 sq. miles (8.752855 sq. km)
Total area (2000) :
504.882952 sq. miles (1307.640787 sq. km)
Located within :
Illinois (IL), FIPS 17
Location :
39.345598 N, 87.802973 W