Guidebook—Geology and Paleontology of Northwestern Kansas

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From Oakley to Stop 1

We begin our field trip in Oakley, at the crossroads of two federal highways, U.S. 40 and U.S. 83. Located in the extreme northeastern corner of Logan County, Oakley is the county seat, replacing the more centrally located former county seat of Russell Springs. Its location on major transportation routes, including I-70 and the Union Pacific Railroad, has allowed Oakley to survive, while Russell Springs has faded in importance.

Oakley is situated in the High Plains physiographic region; the elevation of our starting point is 3,062 feet. Beneath Oakley is up to 226 feet of Ogallala Formation (see stratigraphic column below), the primary component of the High Plains aquifer. As we will discuss in more detail at Stop 3, the Ogallala consists of sediments that eroded off the Rocky Mountains during the Tertiary Period (see geologic timetable).

diagram of rock column showing Upper Cretaceous Smoky Hill Chalk Member of Niobrara Chalk and Pierre Shale, as well as Quaternary Ogallala Formation

Stratigraphic column of rocks encountered on this field trip.

The High Plains aquifer supplies the numerous center-pivots that irrigate the fields around Oakley. Nearly all the irrigation in Logan County occurs along the northern fringe of the county. As we proceed south along U.S. 83 toward the Smoky Hill River, we will see the landscape and agriculture change. The irrigated fields will be replaced by dryland farms and shortgrass pastures and rangeland. Once we enter the Smoky Hills physiographic region, the topography will become more dissected and outcrops more common.

Much of the High Plains is covered by a fine, loose silt called loess. Much of this loess was deposited as windblown dust during the ice ages of the last few hundred thousand years. Although glaciers never reached this part of Kansas, glaciers in the Rockies to the west and in the upper Midwest and northern Plains produced large amounts of finely ground rock material that was carried by strong winds and deposited across large areas, including the High Plains of Kansas.

This loess has buried the Ogallala and older rocks that would ordinarily crop out across the High Plains. Outcrops are restricted to those areas where stream erosion has removed the overlying loess. Our first glimpse of the Ogallala occurs as we cross the Middle Branch of Hackberry Creek, about 8 miles south of Oakley. As we continue south, deeper into the Smoky Hill River valley, we enter an area where the Ogallala has been removed by erosion and the Niobrara Chalk from the Cretaceous Period pokes out from its loess covering.

Our first stop will be at Monument Rocks, where we'll get a close-up look at the Niobrara Chalk.

This guidebook is also available in print form as Kansas Geological Survey, Open-file Report 2002-42, from KGS Publications Sales office, 785-864-3965.

Unless noted otherwise, illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey. Text by Liz Brosius, Jim McCauley, Bob Sawin, and Rex Buchanan, Kansas Geological Survey.

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