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News Release, Kansas Geological Survey, Dec. 21, 2010

Geologic Map for Geary County now available

LAWRENCE--A new full-color geologic map of Geary County with a three-dimensional quality that accentuates the county's predominant Flint Hills terrain is available from the Kansas Geological Survey based at the University of Kansas.

Showing the type and age of rock layers found at the surface or immediately under the vegetation and soil, the map also features infrastructure built since the county was last mapped in 1941. Updates include the addition of Interstate 70 and Milford Lake, constructed in the 1960s.

The new map was produced by Survey geologist Bob Sawin and Kansas State University professor emeritus Ron West.

Over the past several years increased development around Junction City, tied to escalated troop strength at Fort Riley and overflow growth from nearby Manhattan in Riley County, has underscored the need for updated geologic information.

"It's important to understand how geology will affect construction activity," Sawin said. "Using maps, aerial photographs, and technology not available when the 1941 map was made, we were able to locate geologic features that could affect development decisions, such as sand dune areas west of Junction City."

While the dunes were created from sand blown upland from the Smoky Hill River by prevailing southerly winds within the last 10,000 years ago, the bulk of the surface rocks in the county and the entire Flint Hills region was formed from sediment deposited in ancient seas about 275 to 290 million years ago.

The Flint Hills were formed from alternating layers of Permian-age limestone and mudrock now exposed at the surface or in road cuts, and the hills are capped with limestone, which weathers more slowly than mudstone. However, the region got its name from the flint, or chert, embedded in some of the limestone units.

Hilly topography and thin, rocky soils make the uplands difficult to cultivate, but the native grasses they support are good for grazing cattle and other livestock. Crops are grown in the floodplain soils and in some upland locations where soils were formed from silt, or loess, transported from glacial sources northeast of Geary County by wind and water. No glaciers reached Geary County.

Other economic activities sustained by the county's geologic resources include production of limestone for building, road material, and agricultural lime and excavation of sand and gravel from the major floodplains.

Water, mostly in Milford Lake, covers 5% of Geary County, and three of the state's major rivers are found there. The confluence of two, the Republican and Smoky Hill rivers, creates the headwaters of the third, the Kansas River, on the northeastern edge of Junction City. Several tributaries of the Kansas River, including McDowell, Humboldt and Clark creeks, run generally southward through the county amid the Flint Hills.

In addition to rock units and relief, the map shows towns, roads--from federal highways to unimproved roadways--ponds and streams, sand and gravel pits and limestone quarries, and elevation contours at 10-meter intervals.

Drawn in full-color to differentiate rock layers, the computer-generated geologic map features shaded relief to emphasizes the landscape's topography. The map is at a scale of 1:50,000 so that one inch on the map equals about 3/4 mile of actual distance.

Besides the map, the 56" x 38" sheet contains an illustrated rock column, which shows the order in which the rock units were deposited over time, and a description of each unit.

Copies of the Geary County map are available from the Kansas Geological Survey at 1930 Constant Ave., Lawrence, KS 66047-3724 (phone 785-864-3965, email and at 4150 W. Monroe St., Wichita, KS 67209-2640 (phone 316-943-2343, email The cost is $15 plus shipping and handling. Inquire about shipping and handling charges and, for Kansas residents, sales tax. More information about the maps and other KGS products is available at the Survey's web site (

Link of interest to this article:
Geary County geologic map

Kansas Geological Survey, Public Outreach