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Erosion and soil loss in the San Juan Basin

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Mature Ponderosa pine that has been undercut by arroyo incision.
(click for a larger version)
Kevin Hobbs
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Note how the tree's roots preferentially follow weak fractures in the top of the bedrock.
(click for a larger version)
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A landscape that has lost practically all soil cover.
(click for a larger version)

— May 6, 2022

In sandstone-dominated highlands of the San Juan Basin in northwestern New Mexico, many landscapes are in transition from soil-mantled forested slopes to unvegetated slickrock slopes. This process is altering the hydrology and ecology of the region. In the first photographs, a mature Ponderosa pine has been undercut by arroyo incision, showing the depth of soils in the area (about 2 meters) as well as the gradational weathering of the region's sandstone bedrock (in this case, the Ojo Alamo Sandstone). This rock weathering creates a geological product called saprolite that has properties of both rock and soil. As in many environments, there is no distinct dividing line between rock and soil here. Note how the tree's roots preferentially follow weak fractures in the top of the bedrock - an excellent reminder of the important role that plants take in turning rock into sediment! Since this photo was taken in 2014, this tree has toppled into the arroyo.

The final photograph shows a landscape that has lost practically all soil cover. Small trees, grasses, and shrubs still persist in fractures and low spots, but the landscape is largely unvegetated. Summer rainstorms generate intense runoff since the precipitation cannot soak in through the bare rock as it would through a soil. These runoff events lead to further erosion and soil loss, creating a positive feedback loop; i.e., less soil leads to more runoff, which leads to less soil.

— Kevin Hobbs, Field Geologist, NMBGMR