Geologic map for Hodgeman County now available

LAWRENCE — Limestone used for fence posts, sediment washed off the Rocky Mountains millions of year ago and hundreds of natural, intermittent playa lakes of uncertain origin are among the geologic attributes featured on a new full-color map of Hodgeman County in west-central Kansas.

The geologic map, available from the Kansas Geological Survey at the University of Kansas, shows which rock formations crop out at the surface or lie immediately under the soil and vegetation. Widespread deposits of windblown silt and waterborne sand and gravel are also shown.

Produced by KU geography professor William Johnson and KU geography graduate student Terri Woodburn, the computer-generated map includes shaded relief. The three-dimensional quality highlights the two main regions extending into the county — the Smoky Hills from the northeast and the relatively flat High Plains from the west — and differentiates the stream valleys from the uplands.

Rocks and sediment found at the surface in Hodgeman County represent millions of years of geologic processes and range from the Dakota sandstone, formed about 100 million years ago in shallow Cretaceous sea deposits, to windblown silt called loess, which has been deposited over the last 100,000 years.

Greenhorn limestone, slightly younger than the Dakota sandstone, has historically been quarried for buildings and bridges, although it was mainly been used for fence posts. The marine fossil clam Inoceramus is common in the Greenhorn.

The Ogallala Formation, composed of gravel, sand, silt and clay transported by streams from the Rocky Mountains about 5 million years ago, varies from unconsolidated deposits to a cemented material locally known as mortar beds.

The water-bearing portion of the formation, the Ogallala aquifer, is part of the massive High Plains aquifer that underlies parts of eight states and is the main water source for western and central Kansas.

Although numerous streams and small tributaries appear on the map, surface-water resources are limited.

“In Hodgeman County, baseflow in the Pawnee River and smaller streams is low, and sometimes nonexistent, except during significant precipitation events,” Johnson said. “As a result, average flow from year to year is highly variable, and the county depends mainly on groundwater for its municipal, industrial, and agricultural needs.”

Most streams in the county, including the Pawnee River along the northern border, Buckner Creek running west to east through the middle, and Saw Log Creek in the southeast, are in the Arkansas River drainage basin.

The county’s two incorporated towns, Jetmore and Hanston, are located on Buckner Creek, and three reservoirs — Horse Thief Reservoir, Jetmore City Lake and Hodgeman State Fishing Lake — have been built on Buckner Creek or its tributaries.

Playa lakes, transient wetlands fed only by precipitation and runoff, are scattered on the uplands away from the stream valleys throughout the county. When wet, the small, shallow basins — also known as lagoons or buffalo wallows — help recharge the aquifer and provide refuge for local and migratory wildlife.

In addition to geologic and hydrologic characteristics, the map includes towns, roads (from federal highways to unimproved roadways), elevation contours at 10-meter intervals, and township and range boundaries.

The map, one of a series of county maps that cover the state, is drawn at a scale of 1:50,000 so that 1 inch on the map equals about 3/4 mile of actual distance. Besides the map, the 63" x 40" sheet contains an illustrated rock column showing the order in which the rock units were deposited over time and a description of each unit.

Copies of the Hodgeman County map are available from the Kansas Geological Survey at 1930 Constant Ave., Lawrence 66047-3724 (phone 785-864-3965, email pubsales@kgs.ku.edu) and at 4150 W. Monroe St., Wichita, KS 67209-2640 (phone 316-943-2343, email wwsl@kgs.ku.edu).

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 website.