Many geologic features that are common on Mars are less common or less well understood on Earth, so learning more about these features helps us to understand both planets. River valleys on Mars are often described as "stubby" due to their steep sidewalls and rounded, theater-shaped headwalls. On Earth, these valleys are often called box canyons, because you have to climb out near the valley head or go back out the way you came. Theater-headed valleys are often attributed to groundwater sapping, where spring flow weathers and erodes the base of the headwall, causing the top to collapse repeatedly over time. However, other geologic processes including flood erosion of strong layers over weak ones and flood erosion of rock with vertical fractures have been suggested as possible causes.
Our field study includes theater-headed valleys in Utah and the Atacama Desert of northern Chile. We are also examining the largest springs in the U.S. to compare channels formed by springs with those formed mainly by flash floods. In Chile, we found that flood erosion of strong-over-weak layers, helped by vertical jointing of the strong top layer, was responsible in some cases. However, giant landslides appear to have formed other wide theater headwalls. In Utah, spring flow is actively weathering the Navajo sandstone, but flash floods are responsible for most of the erosion of weathered debris. Channels formed by spring flow are not much larger than the stream under low-flow conditions, and the woody debris that falls into them is rarely washed out. In contrast, flash flood channels are much larger than the spring flow, and flash floods are capable of moving much larger rock or woody debris.
Figure: (a) Theater headwall of Fence Canyon in southern Utah. (b) Theater-headed valley in the Atacama Desert, Chile, looking down-valley from the top of the headwall. (c) Bottom of the same headwall. Note the difference in vegetation between headwalls with springs (Utah) and without them (Chile).