The interpretation of geological observations can be difficult. It is unlike any biological, physical or chemical experiment in the laboratory, which is usually more straightforward. In geology, fieldwork involves observations of structure, rock type and fossils, etc. But no one can run an experiment to observe how these particular features were formed. The data must be interpreted, and interpretations are based on the framework of one’s worldview or beliefs about the past. But regardless of worldview, seemingly straightforward geological deductions can still be wrong; further data may bring to light an alternative explanatory mechanism.
Erosion of Box Canyon likely not from sapping
Such an explanatory switch recently occurred in interpreting the geological structure of Box Canyon, cut into the Snake River Basalt in south central Idaho.1 The Canyon has vertical walls 35 m high, 2.68 km long and 120 m wide, with a sinuous longitudinal profile that opens up into the Snake River Canyon. Box Canyon has been explained by sapping erosion, where water seeping horizontally out of a permeable layer erodes the basalt above, causing blocks of this harder layer to tumble down. Such a process occurring over a considerable period of time results in an amphitheater-headed canyon. In fact, such box canyons have been considered as a diagnostic tool for determining erosion by sapping, especially on the Colorado Plateau.
Box Canyon, Idaho, has been considered a classical canyon carved by sapping because it is incised into the basaltic plain and has no upstream drainage network. Approximately 10 m3 s-1 of seepage currently emanates from the headwall, and it is the 11th largest spring in theUnited States. Sapping seems like a shoehorn explanation for the canyon. Unfortunately, sapping erosion has been demonstrated only in unconsolidated sand (figure 1).2Therefore, sapping erosion is really an inference when applied to hard rocks:
“ … we know of no unambiguous case of seepage eroding an amphitheater-headed valley in resistant rock, several examples exist of valley formation by runoff and mass wasting processes in the absence of seepage erosion.”….
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