   marion county is a county located in the u.s. state of illinois. according to the 2010 census, it had a population of 39,437. its county seat is salem. marion county comprises the centralia, il micropolitan statistical area, which is included in the st. louis-st. charles-farmington, mo-il combined statistical area. == history == marion county was organized on 24 january 1823 from portions of jefferson and fayette counties. it was named in honor of revolutionary war gen. francis marion, the "swamp fox". file:marion county illinois 1823.png|marion county at the time of its creation in 1823 ==geography== according to the u.s. census bureau, the county has a total area of , of which  is land and  (0.6%) is water. the southwest corner of marion county is the intersection of the baseline with the third principal meridian, the point of origin for the third survey of the northwest territory under the land ordinance of 1785. the origin is marked with a boulder south of centralia just off u.s. 51. ===climate and weather===  in recent years, average temperatures in the county s
1823_establishments_in_Illinois Illinois_counties Little_Egypt Marion_County,_Illinois Populated_places_established_in_1823