Erosion surfaces in the Española basin formed before 350 ka and between 350 and 240, 240 and 130, and 130 and 80 ka, probably in response to climatic change and regional uplift. The surfaces are cut on Miocene, Pliocene, and Pleistocene deposits and range from about 200 m to 15 m above the present Rio Chama/Rio Grande system. Periods when the surfaces formed were dated using varnish-cation ratios from exposed clasts, the mass of soil carbonate, and amino-acid ratios in Pleistocene gastropods from underlying deposits. Thorium/uranium ages from soil carbonate were used to calibrate a local curve for varnish-cation ratios. The range in age determined for a given surface, although derived from different dating techniques, implies that parts of the surface were sites of erosion or aggradation after the surface formed.
From 1.1 Ma to present, denudation rates averaged 10 cm/1,000 yr from weakly lithified sandstone, less than 7 cm/1,000 yr from indurated tuff and boulder gravel, and about 4 cm/1,000 yr from tuff and basalt. Erosion surfaces were preserved as upland benches and terraces by stream incision during periods of pluvial climate and regional uplift, but our data do not permit clear separation of the two causes.