April 14, 2021
"I have been mapping the Santa Fe Group and post-Santa Fe Group deposits near San Marcial over the past year. In December, I came across this neat exposure of axial sand laid down by the Rio Grande about 2-3 million years ago. The first photo shows very thin-thin beds of volcanic pebbles interbedded in the white, medium-grained river sand. Above the blue pen are a couple of spheroidal clay balls. These balls formed when the river ripped up a pre-existing, hardened clay bed (presumably on a floodplain). The ripped-up clay rolled downstream like a piece of gravel, becoming rounded in the process. If the clay ball rolls across pebbles on the bed of the river (like the pebble beds seen below the pen), then the pebbles can stick to the clay -- like rolling a ball of cookie dough over chocolate chips and having the chips stick to the outside of the ball-of-dough. An example of such an "armored clay ball" is seen in the second photo, with a blue pen for scale. The clay ball in the second photo may actually be comprised of smaller clay balls that have been compressed together to form one larger clay ball, which was later armored."
— Dan Koning, Sr. Field Geologist, NMBGMR