22 September 2011
More moki marbles: little concretions in sandstone, kind of like the ones I showed you Tuesday from Illinois.
But these ones are from the Navajo Sandstone, a late Triassic or early Jurassic erg deposit from the Colorado Plateau.
These photos were taken in Zion National Park, near Springdale, Utah (real close to the cross-beds I featured a month ago).
They are tougher than the sandstone in which they formed, and because they stand up better to the vagaries of weathering, the surrounding rock gets etched away more rapidly, and the little concretions stand out in high relief.