Continuous soil-moisture measurements to assess fracture flow in Inscription Rock at El Morro National Monument, New Mexico: Implications for the deterioration of inscriptions
— B. Talon Newton and Shari A. Kelley

Inscription Rock, the main attraction at El Morro National Monument, is a 70-m-high sandstone monolith that documents a long history of human activity. The Atsinna Pueblo ruins sit on top of the monolith above steep cliff walls that exhibit thousands of signatures carved into the sandstone by Spanish explorers and American emigrants. One of the primary goals of the National Park Service in managing the monument is the preservation of the historic inscriptions, which are deteriorating largely due to natural weathering processes that are driven by the presence of water. The loss of inscriptions is particularly noticeable on the northeastern-most point on the cliff at El Morro, where lichen is obscuring inscriptions. The experiment described here was part of a larger hydrogeologic study with the objective of identifying water sources and mechanisms by which water comes in contact with the inscriptions.

This experiment compared fluctuations in soil moisture at the base of the cliff in different areas to soil moisture fluctuations at “control points” away from the cliff, where local precipitation that falls on the surface is most likely the only source of soil moisture. Continuous soil moisture data at depths of 10 and 30 cm were collected between April 27 and October 5, 2017. Soil moisture data for sites in close proximity of the cliff showed evidence of additional soil moisture sources in areas where the cliff face was perpendicular to the northeasterly strike of the primary joint system, providing evidence of water percolating relatively quickly through these fractures. Water that is stored in these fractures that are close to the wall surface can potentially move slowly through the sandstone matrix, mainly driven by capillary action and a water potential gradient resulting from the evaporation of water that reaches the rock surface. We hypothesize that this process is occurring on the north side of Inscription Rock, where lichen growth is greatly impacting inscriptions.