Socorro County, NM
— May 17, 2021
The first photo shows a desert pavement developed on a terrace surface in the Fort Craig area. There is a blue pen near the center of the photograph, just on the other side of the gully. Note that beneath the strongly varnished gravel lies a 10-20 cm-thick layer of gravel-lacking, fine-grained sediment. This fine-grained layer overlies a gravelly terrace whose surface stands 20 ft above the adjoining active arroyo floor. Desert pavements consist of a surface-armor of gravel clasts that overlie a certain type of soil structure called Av peds (second photo, but from a different location); the word "peds" more or less corresponds to what most people call "dirt clods." The Av peds are silty-clayey and have little bubbles formed by gas expansion when the soil heats up after a summer rainstorm event. You can see the little bubbles in the dirt clods above the pencil in the second photograph. A model proposed for the development of desert pavement (by Les McFadden and Stephen Wells, the latter now president of NM Tech) is that the silty-clayey, fine-grained layer slowly builds up with time due to accumulation of eolian clayey silt. The gravel armor rides on top of this accreting mantle of fine sediment (for more details, take a look at McFadden et al. in Geology, v. 15, p. 504-508)." I find desert pavements fascinating, and when I map them tend to keep close tabs on their properties. Besides the scientific interest, they are a bit sentimental for me because my first geology job was to map desert pavements in the Mojave Desert of Southern California.
— Dan Koning, Sr. Field Geologist, NMBGMR