Southwest Arkansas, the area where I wrote my geologic master’s thesis, is rife with structural faults that extend all the way to the surface. Still, for those that have visited southwest Arkansas, they know that trees and undergrowth there cover practically everything. This means that surface faults are difficult to locate and to map. Modern technology such as side-looking radar, Landsat, and even aerial photos help alleviate this problem, but what did early explorers and prospectors do to locate and map these faults before this technology existed? Well, they consulted Mother Nature.
You gardeners know that plants and trees prefer specific chemistries in the soil they occupy. Pine trees like a particular acidity, oaks something totally different. Smart prospectors and savvy field geologists still use this very basic bit of information to their advantage.
How does it work? In southwest Arkansas, many near-vertical reverse faults often extend to the surface. For those that understand about the structural mechanism of faulting, they know faults often separate rock of different ages and chemical composition. Side "A" might comprise an acidic soil, Side "B" more alkaline. Pines, that may prefer a more acid soil, would grow to the very edge of the fault on Side "A", and then stop. Oaks like more alkaline soil and would prosper on Side "B" right up to the edge of the fault line. The result is an abrupt change in vegetation that alerts explorers that a surface fault is present.
Prospectors and field geologists are trained to notice these "tree-line faults." Next time you are on a field trip, check it out.
