Septarian Nodules

 The name “septarian nodules” is derived from the Latin word “septum,” meaning seven, in reference to partitions dividing cavities. More specifically, the name refers to carbonate-rich nodules having an internal structure comprised of a series of carbonate-filled cracks or cavities, which separate polygonal blocks of hardened sedimentary material, such as mudstone.

The exterior of a septarian nodule, 13 cm tall and 11 cm wide. Utah.

Septarian nodules on the outside are not exactly eye-catching.

The interior of the same nodule, shown above. The interior of a septarian nodule can be, however, very interesting and eye-catching.

The nodules usually form early in the burial history of the muddy sediment before the rest of the sediment hardens into rock. Afterward, compaction causes cracks/cavities to form, and groundwater with dissolved minerals infills the cracks/cavities. Upon evaporation, the minerals crystallize out of the water. 

Septarian nodules, more accurately referred to as septarian concretions, have a smooth rounded exterior, which is normally gray. The mineral-filled cracks in the interior are typically yellow (calcite), but some also can be partially brown (aragonite = another carbonate mineral).

This septarian nodule (12 cm wide and 15 cm tall) was cut (slabbed) in order to show the intricacy of the infilled cavities.

Lastly, I included this slabbed (cut) septarian nodule because it is a nice specimen and one that was used in one of my previous posts (July 17, 2014) entitled "Pseudofossils."The rounded and smooth exterior of most of the septarian nodules have been mistakenly by some collectors as “dinosaur eggs,” which they are NOT! Pseudofossils are inorganic objects.

Rock hounds commonly cut the septarian nodules in half and polish them. They can sell for low to high prices, depending on the complexity of the radiating crack pattern and the variety of the coloration.

Mud curls that resemble arthropod appendages

In the early 1980's, geologists and paleontologists were excited by the discovery of what appeared to be arthropod appendages (legs) preserved in a slab of upper Precambrian or lower Cambrian silty sandstone found in the Grand Canyon, Arizona. Pictures of the slab were published on the cover of at least one leading journal (I searched the internet but could not find the article).

The slab was put on display at some major universities in the American west. I viewed the slab at Caltech.

A bedding-plane view of the slab is shown below (top image). The slab is about 2 feet in width and seemed to be very heavy. A cross-section view of the slab is shown below (bottom image).

The significance of this slab is that, if it actually contains fossil arthropod appendages, then they were larger and more evolved than previously believed. Although the bedding-plane view seemed to support the idea that the structures were "legs" of some sort, the cross-section view did not support this idea. Cooler heads prevailed eventually, and most experts ruled that these structures are actually "mud curls" or "sun curls," caused by the shrinkage of muddy layers on an ancient shoreline or lake. In summary, the slab contains pseudofossils, not fossils.

Once the geologic community, as a whole, considered the evidence, this slab became relegated to the dark depths of some storage room and was not mentioned again!

It is interesting to mention that *McKee (1932), a Grand Canyon Park naturalist, illustrated slabs similar to the one shown above. He referred to them as "fucoides," a name used by paleontologists then to describe any unusual "biologic?" structure, like algal remains, "worm" burrows, etc.

*McKee, E.D. 1932. Some fucoides from the Grand Canyon. Grand Canyon Nature Notes 7(8): approx. 5 pp.  [pdf available online for free].