Core to map: Scouting for outcrops
Editor’s note: The E-Geo News has been walking readers through the various steps required to turn physical data such as core samples into a functional geologic map. This is the fourth installment in this series; core collection was covered in September, core describing in November, and lab analysis in December.
It’s a warmish, sunny, January Tuesday, and Research Geologist Isaac Allred is on a working lunch. His office: The tucked-away trails of Hemlock Cliffs nature preserve in Crawford County. His objective: finding the point in this bedrock valley where two distinct ages of rock converge.
IGWS geologist Isaac Allred explores the contact between the Stephensport and Buffalo Wallow groups under a waterfall at Hemlock Cliffs in Crawford County. | Sara Clifford, IGWS
Rock hammer in hand, he finds what he’s looking for less than a mile in, clambering up and under a waterfall in a shallow canyon and recording the “meeting” with a photo. Tar Springs Formation sandstone above us, Glen Dean Limestone below us, this a group contact, a data point important to structural engineers, natural resource seekers (building stone; aggregates like gravel and sand; oil and gas; and water), zoning directors, and others.
On the other end of the trail, Valerie Beckham-Feller and Don Tripp work their way up from underneath another sun-dappled waterfall with a pole and a sighting device, measuring how many “Dons” it takes to get from the base of the Tick Ridge Sandstone Member of the Tar Springs Formation to its contact with the Glen Dean Limestone. (Answer: 14 Dons, or about 71.4 feet of layer thickness.) Thicknesses of formations—of sandstone, shale, and limestone—within rock unit groups are other major data points plotted on bedrock maps.
IGWS researchers Don Trip and Valerie Beckham-Feller measure the thickness of the Tick Ridge Sandstone Member at Hemlock Cliffs in Crawford County. | Sara Clifford, IGWS
When making a map, IGWS geologists not only gather new subsurface data through core drilling; they also pore through legacy data gathered over the past 100-plus years by other Survey geologists, as well as by oil and gas exploration companies. This area of the state—the Jasper quad—has been of limited commercial interest, so the team working on a new map through the U.S. Geological Survey’s STATEMAP program has primarily been verifying and adding to data collected by their predecessors—Clyde Malott, Henry Gray, and others—between the 1920s and ‘50s.
The Jasper team plotted places they wanted to visit on a digital map—some to verify data collected with less precise methods, some to gather new data in areas where the stratigraphy had been projected rather than measured. When some of this legacy data was collected, field geologists used a clinometer (an instrument to measure slope), a barometer, and a known altitude, like at a county courthouse, to plot locations and extrapolate thicknesses. In Crawford County, Tripp opened an app—which auto-locates him on GPS if he’s in service—entered notes about the elevation; the rock unit (group, formation, and member); anything notable he saw in samples, like fossils; and synced the program when he got back to the office. All that information then flows into a database that feeds the mapping software.
Of course, he takes paper notes, too, drawing diagrams of outcrops and noting details such as grain size, color, texture, thickness, and rock type. Even rocks within the same stratigraphic unit can vary from place to place because of changes they may have undergone after they were deposited, like the chemical qualities of the local groundwater that soaked into them over millions of years. Because of this, the Salem Limestone, for instance, could look different in Lawrence County than it does in Harrison County. Field visits help to inform and constrain these ranges of physical attributes.
“Engineers hate us because we’re not exact, but that’s just the nature of the game,” Tripp said.
Being attuned to the details of nature—from canyons and cliffs to sand grains and shells—is part of it, too. While vehicles whizzed by them on a highway shoulder, Allred, Beckham-Feller, and Tripp took turns with hand lenses and samples they’d chipped from a roadcut, searching for “forams” (foraminifera), gastropods, and other tiny fossils that give clues as to the rock’s age and stratigraphic placement.
“That’s, I think, the beauty of geology, is this lifetime search for answers about the Earth,” Allred said. “It’s like putting together a 1,000-piece puzzle. But every single piece can be valuable. … And we continually leverage technology and tools so we can frame this image in better detail.”