Mapping Sends Survey Geologists to the Field

"I want to know what kind of cake is under all this frosting," said Jim McCauley, Kansas Geological Survey geologist, referring to the bedrock or solid rock that lies underneath the soil.

CONTENTS

Geologic Mapping–page 1

From the Director–page 2

New Publications–page 3

Education Award–page 3

A Place To Visit–page 4

Looking out on the Kansas countryside, most people notice the colorful grasses, weathered barns, hawks perched on fenceposts, whatever catches their eye and interest. Geologists, however, look at the landscape differently. Instead of focusing on what’s visible at the surface, they often try to visualize what lies beneath it.

“I want to know what kind of cake is under all that frosting,” said Jim McCauley, Kansas Geological Survey geologist, referring to the bedrock or solid rock that lies underneath the soil. He said he’s always looking for clues about the bedrock. Sometimes the clues are in plain view, as in rocks laid bare by a roadcut. Often they’re more subtle—the slight bump on a gravel road that signals exposed bedrock, the color and thickness of the soil. Taken together, these clues provide a picture of the mostly hidden bedrock.

McCauley is part of the Survey’s geologic mapping program, which produces maps of bedrock in the state’s counties. Because geologic maps show the rock formations likely to be encountered in a given location, they are useful in construction, in understanding soils and agriculture, in searching for water and mineral deposits, and in a variety of engineering and environmental applications.

Although most Kansas counties have modern geologic maps, a handful are being mapped for the first time. Since 1991, the Survey has released new geologic maps for Finney, Phillips, Butler, Riley, Chautauqua, Russell, Montgomery, Ness, Stafford, and Elk counties. McCauley recently finished mapping Leavenworth County and is currently at work on maps of Wyandotte and Comanche counties.

Mapping a county’s bedrock, most of which is covered by soil and vegetation, involves both painstaking sleuthing and a thorough knowledge of the local stratigraphy, the sequence of rock layers found in a given area. Some information about a county’s geology can be gleaned from published materials—old maps, aerial photographs, soil surveys. But, said McCauley, “there’s no substitute for getting out in the field.”

Though technology has changed how some geological research is conducted, McCauley’s approach to mapping is essentially low tech. Generally he works his way across the county stratigraphically, starting with the uppermost (and youngest) rock unit and tracing it throughout the county before going on to the next unit. “This way I get really familiar with the characteristics of the unit,” he said. Generally, unit boundaries represent a change from one rock type to another—for example, from a shale to a limestone. Out in the field he tries to locate as many of these boundaries as possible.

Armed with a stack of U.S. Geological Survey topographic maps (from 15 to 30 depending on the county’s size), a hand level, and a small vial of dilute hydrochloric acid (if it fizzes, it’s limestone), McCauley drives the county roads, scanning the landscape for clues about the underlying rocks. “I always keep an eye on the ditches,” McCauley said. Because ditches are constantly being eroded, they are a good place to look for exposed rock, as are streambeds and gullies. He also scans hillsides, looking for breaks in the slope, which signal a change in the underground rock. Vegetation can also provide clues about the bedrock: oaks, for example, tend to grow in soils formed from sandstones. Manmade exposures such as roadcuts and quarries are good places to examine the bedrock in detail. In urban areas, construction sites can be an important source of information about the bedrock in a largely altered landscape. “You take whatever markers you can find,” McCauley said.

Once a rock unit has been identified, its location is pinpointed on the topographic map, along with its elevation. Each of these locations becomes a data point for the rock unit that’s being mapped. Eventually these points are connected with a line that represents the contact between rock units (which show up on the final geologic map as different colored areas). Because outcrops may be scarce, geologists must fall back on their experience and knowledge (for example, that rock units in Kansas generally dip gently to the west) to fill in the blanks.

This has led critics to claim that geologic mapping is more art than science. McCauley admitted that, in the absence of good exposures, geologists are often forced to make inferences. “But these inferences are preceded by a lifetime of observation and study of our subject and are based on the recognition and interpretation of the form, pattern, and structure that we see in our subject.” With that in mind, he’s willing to stand by the maps. “Plus,” he said, “they make great wall hangings.”

Online February 10, 2003

Comments to: lbrosius@kgs.ku.edu

Kansas Geological Survey

URL:http://www.kgs.ku.edu/Publications/GeoRecord/1999/vol5.1/Page1.html