Moqui Marbles, Zion National Park, 2014.
Concretions are hard, compact masses of matter formed by precipitation of mineral cement within the spaces between particles. They form within sedimentary strata that has already been deposited before the rest of the sediment is hardened into rock. Concretions are found in a variety of rocks, particularly shales, siltstones and sandstones. Moqui Marbles, or Moqui Balls, are iron oxide concretions found in abundance in the Navajo Sandstone of Utah. They are created by the precipitation of iron which was dissolved in groundwater.