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Mason County
Low elevation: <435 feet above mean sea level
Place: Illinois River
7.5' quadrangle name: Clear Lake
PLSS: 3-5-19N-11W
Geology: Cahokia Alluvium in river valley
High elevation: >730 feet above mean sea level
Place: 2 unnamed hills along a ridge
7.5' quadrangle name: Easton
PLSS: 3-25-20N-7W
Geology: Parkland sand ridges (esker?)
Area: 557.44 square miles
Total relief (approximate): 295 feet
Average percent slope: 0.54%
State of Illinois | Illinois Department of Natural Resources | University of Illinois