Erosional history and soil development on Quaternary surfaces, northwest Espanola Basin, New Mexico
— David P. Dethier and Karen A. Demsey

Extensive late Cenozoic fans and pediments form surfaces that flank the Rio Grande River along much of its course in New Mexico. The surfaces record pauses during general downcutting by the Rio Grande, as well as episodes of aggradation. Surface formation is likely linked with tectonic control of base level, but climate change may also be important. Where ages can be determined for pediments and fans, these features can be used to help constrain local uplift and denudation histories. However, easily dated material is uncommon in the arid and semiarid regions of the West, and radiometrically dated Quaternary sequences are rare.

Arid and semiarid basins with axial drainage display a general sequence of landforms outward from flanking mountain ranges: mountain front, pediment or fan, coalescing pediments or fans, and stepped alluvial terraces along the major drainage. In most semiarid regions, pediments and adjacent alluvial fans have similar gradients and transport similar material by flow in one or more channels (Moss, 1977). Surfaces are more likely to be erosional near the mountain front and depositional near the axial drainage, but any surface may display evidence for contemporaneous erosion and deposition. Grain size in many active alluvial fans decreases away from the mountains (Bull, 1964), and preservation of stratigraphic relations and soils is more likely downslope. However, once a surface is abandoned by base-level lowering, lateral erosion may proceed more rapidly in downslope areas where surfaces are covered by finer material. Preservation of a complete stratigraphic record is thus unlikely at any point, but comparison of morphostratigraphic and soil relations on several surfaces aids in reconstruction of geomorphic history.