The normally cream-colored Navajo sandstone is home to large collections of black, round concretions that seem entirely out of place. Similar, but smaller, marble-like formations have been found on Mars. Where do they come from? Science has recently solved the puzzle.
In several places in the Escalante Canyons, hundreds of black stone balls collect on the surface or in the eroded grooves of normally white or cream-colored Navajo sandstone. In other places, bumpy or corrugated sheets of similar black stony material lay on the sandstone or is eroding from it.
The black, rocky sheets are locally called ironstone. The black stone balls are known as Moqui (Moki) marbles. Ironstone is a mix of sand and iron-containing minerals (hematite and goethite). Moqui marbles are comprised of a concentric shell of these same materials surrounding a sandy core.
Thousands of Moqui marbles lie on the Navajo sandstone from which they eroded. Photo: © Donald J. Rommes.
A large number of Moqui marbles are found in the Navajo sandstone expanse of the Red Breaks in Grand Staircase-Escalante National Monument. Hundreds of these round black stones can be seen lying in groups on the surface of the sandstone. Others can be seen encased within the sandstone, as if they are erupting from the rock.
Clear examples of the latter phenomenon can be seen in sandstone walls of washes and streams, where Moqui marbles appear to be in all phases of “escape” from the rock—from a rounded bump fully covered by sandstone, to a black ball stuck in the rock like a baseball thrown into tar. In other cases, only a concave half-sphere remains, the sandy core having been washed out.
When I first studied these Moqui-marble-embedded walls, I imagined a whole population of sentient spheres, encased in stone for eons, struggling to work themselves out of their rocky prison. Finally doing so, they then roll giddily downhill to gather with others of their kind.
In the lower center of this photo, an empty crater can be seen that formerly held a Moqui marble. Many other examples of Moqui marbles in various stages of eroding from Navajo sandstone can be seen in this slot canyon. Photo: © Donald J. Rommes.
The Hopi (called the Moqui by the Spaniards) had other thoughts. According to some sources, they imagined the spirits of their ancestors playing games with these marbles in the evenings.
More prosaically, scientists tell us the formation of the Moqui marbles has more to do with chemistry. Sand, the raw material for Navajo sandstone, was comprised of iron-bearing silicates. Gradually, the iron was dissolved by a reducing environment (probably petroleum-induced) as the sand was compacted into stone. Much later, groundwater flowing through the porous sandstone created an oxidizing environment that forced the iron to precipitate out of solution to fused into these concretions. A study of the locations of Moqui marbles thus provides an understanding of the historical flow of groundwater.
Since this is a secondary phenomenon, Moqui marbles are much younger than the rock in which they are embedded. Navajo sandstone was formed about 180 million years ago, but Moqui marbles are less than 30 million years old and may be as young as 300,000 years of age.
If you think about it, Moqui marbles may also hold the clue as to why some of the Navajo sandstone is red (around The Gulch, for example) while most of it is white.
More on that later.