February 11, 2022
Geologists often find themselves hiking up, over, and down hills, peaks, and valleys in straight lines – despite the topography – to get to our area of interest. On this day, however, geologic mapping near Gallup revealed this natural stairwell that made for a leisurely ascent up tilted beds of the Late Cretaceous Twowells Tongue of the Dakota Sandstone. Most likely, dissolution of calcium carbonate in the rock led to the small pits found across the outcrop, which were further accentuated by chemical and physical weathering processes from surface water runoff and wind to carve out the channel and form the steps.
Looking at the outcrop from afar in the second picture shows us why water and wind were focused to cause erosion in such a uniform straight line (the light yellow box shows the area of the first picture). Lines are observed in the grayish white to yellow rock (Twowells Tongue) that systematically transect the outcrop left-to-right and up-and-down. These are a type of rock deformation feature known as joints (fractures). Joints are not faults, since rock on either side of the joints do not exhibit motion relative to each other; they are basically simple cracks in the rock. The red rocks at the skyline are the main body of the Dakota Sandstone and show a similar systematic array of joints, but oriented in a slightly different and singular direction. The presence of these features were a fundamental characteristic of the rock that led to its present appearance now exposed at Earth’s surface.
The importance of joints goes beyond their role in making a unique rock formation. Similar to how these joints controlled surface water runoff at Earth’s surface, joints still buried in the subsurface can provide permeable pathways for fluids. Therefore, by mapping geology at Earth’s surface, we can also make important inferences about what lies beneath our feet.
— Jacob O. Thacker, Field Geologist, NMBGMR