SURFICIAL GEOLOGY OF HODGEMAN COUNTY, KANSAS

Hodgeman County is in west-central Kansas on the eastern edge of the High Plains and the southern edge of the Smoky Hills physiographic region. The High Plains here consist of loess- and Ogallala-capped uplands, while the Smoky Hills consist primarily of Cretaceous Carlile Shale and Greenhorn Limestone uplands and intermediate slopes. This surficial geology map shows bedrock layers at the surface or immediately under vegetation and soil. It also updates infrastructure built since the county was last mapped (1932), including Horse Thief Reservoir, completed in 2010.

Cretaceous (145-65 MYA) and Neogene (23-2.6 MYA) sedimentary rocks - sandstones, limestones, shales, cherts, and conglomerates - crop out in the county and range in age from Lower Cretaceous (Dakota Fm) to the Miocene and possibly early Pliocene (Ogallala Fm). Pleistocene and Holocene loess (wind-deposited silt) mantles most of the southern and eastern uplands of the county. Topographic relief in the county is 181 m, with the highest area (816 m) in the northwest corner where the Fort Hays Limestone crops out, and the lowest (635 m) in the northeast corner, where the Pawnee River, the county’s primary drainage, exits Hodgeman County. Surface streamflow of the major drainages in Hodgeman County is characterized by very low to no baseflow, except during significant precipitation events, and by high variability in yearly averages, ranging from no flow to high flow.

Mineral resources in Hodgeman County include sand and gravel from the Ogallala Fm and alluvial deposits, building stone quarried from the Greenhorn Limestone, and oil and gas. Oil and gas production is concentrated in subsurface Paleozoic rock formations located in all but the southwestern third of the county.

The map shows the distribution, rock type, and age of bedrock. It can be used to identify surface and subsurface lithologic units and their stratigraphic relationships, show geologic structures, delineate thick surficial materials such as alluvium, and determine the features’ spatial orientation. It includes a stratigraphic column depicting the vertical sequence, thickness, and lithologies of the geologic units, and generalized descriptions. An east-west cross section shows the vertical relationship of the rock units. The geology was mapped on USGS 7.5’ quadrangles and completed in ArcGIS.