By Rex Buchanan, Director Emeritus, Kansas Geological Survey

For the most part, the Lawrence Loop follows fairly flat topography and, like most roads, avoids abrupt changes in elevation. As a result, the trail encounters few roadcuts where the local geology is visibly exposed.  One clear exception to that, however, is the western stretch of the loop that runs through the floodway for Clinton Lake.  There, just to the northeast of the trail, the local geology is on full display.

The outcropping rocks here are interbedded limestones and shales, rocks that demonstrate the cycles of deposition that occurred about 300 million years ago, during the Pennsylvanian Period of geologic history (also known as the Coal Age), when this area was near the equator and covered by a warm, shallow sea.  That sea receded, then recovered the area repeatedly, leaving behind these alternating rock types.

Each of these rock layers has a name.  The two major rock layers here are the Oread Formation (named for the hill where KU is located) and the Lawrence Formation (named for the city of Lawrence).  Each of these formations is further subdivided into rock layers called members (somewhat the way plants and animals are classified by genus, then subdivided into species).

Toward the base of the floodway, a layer of limestone called the Amazonia Member of the Lawrence Formation is exposed.  It’s named for a small town north of St. Joseph, Missouri, where it also crops out.  Above the Amazonia is the Williamsburg coal bed (named for Williamsburg, Kansas), also a part of the Lawrence Formation.  Thin layers of coal are fairly common in Douglas County; the one here is just barely visible.

Above the Lawrence Formation is the Oread Formation, the lowest member of which is the Toronto limestone (named for Toronto, Kansas, and not the city in Canada).  The Toronto is a thick, prominent, brown rock layer here.  While many of the Pennsylvanian limestones in eastern Kansas contain lots of fossils (evidence of the shallow sea that was here when they were deposited), the Toronto is fairly devoid of fossils.  Above the Toronto limestone are the Snyderville shale and Leavenworth limestone members of the Oread.

Probably the most easily identifiable rock layer here is the Heebner Shale Member of the Oread.  The Heebner is very dark, even black.  It is thinly bedded, breaking apart in sheets that are almost like pages in a book.  Within those sheets are small, marble-like rocks called phosphate nodules.  These nodules are very slightly radioactive, and geologists measure and use that radioactivity where the Heebner is in the deep subsurface, like western Kansas, so that they can determine where they are stratigraphically.

Above the Heebner is the Plattsmouth Limestone Member of the Oread.  It is wavy-bedded and contains common Pennsylvanian invertebrate fossils like crinoids, corals, or fusulinids.

Geologists believe that these interbedded limestones and shales are evidence of climate change from long ago.  When the environment was warmer, and polar ice caps melted, sea levels might have risen, leaving behind limestone.  When the climate was cooler, and ice caps grew, sea levels dropped and rocks like shale or coal might have been deposited.  There is, however, disagreement about aspects of this cyclicity and some rocks, like the Heebner, might be evidence of deep-water and not shallow water deposition.  Regardless, the climate change that created this cyclicty was obviously not the result of human activity, like today’s climate change, because these rocks were deposited long before humans existed.

In addition to Pennsylvanian rocks, the Lawrence Loop is near evidence of more recent geologic activity: glaciation. About 600,000 years ago, glaciers moved into northeastern Kansas, stopping at about the Wakarusa River on the south and the Big Blue River on the west.  Geologists believe that this ice was as much as 500 feet thick in today’s Lawrence.  Because so much time has elapsed since that glacial advance, relatively little evidence of its presence remains obvious today (as opposed to areas in Wisconsin and Minnesota, say, where evidence of much more recent glaciation is readily visible). However, large boulders of red quartzite, along with other glacial debris, were left behind here, including some near the Mary’s Lake area, just east of the Lawrence Loop. This would have been close to the glacier’s southern terminus. 

For more information about the floodway’s geology, see Stop 1 in “Geology of Northeastern Kansas Field Trip,” by Jim McCauley, Liz Brosius, Rex Buchanan, and Bob Sawin, Kansas Geological Survey Open0-file Report 2000-55, 2000. On-line at http://www.kgs.ku.edu/Publications/Trips/Reports/OFR2000-55.pdf

For more information about glacial geology in Kansas, see http://www.kgs.ku.edu/Publications/PIC/pic28.html