Mysterious “Cannonballs” Surface in Albany County

In central Albany County there is a geological feature that could easily fool laypersons into thinking they have stumbled upon an old military storehouse, dinosaur eggs, or a fascinating collection of geodes.

 However, these round objects varying in size are actually “calcareous concretions”, and are the result of localized precipitation of calcium carbonate within a layer of sedimentary rock. Unlike geodes, which are usually round objects that are hollow in the center with the interior edges glistening with crystals, these “cannonballs” are entirely rock and may have a center core of a piece of hard sandstone or a fossil fragment.

 Some are found about 5 miles east of Rock River, where 80 million years ago the area was a nearshore environment of the Cretaceous Western Interior Seaway. Today geologists refer to these rocks as the Rock River Formation, part of the widespread Mesaverde Group. The “cannonballs” formed in place in the rock, cemented in concentric layers usually around a resistant nucleus such as a fragment of fossil or a pebble. Because of this cementation, they resist erosion better than the softer sandstone or siltstone that they are embedded in.  Erosion in the present-day outcrop has freed these concretions from their host layer. 

They are especially evident in a place called “Cannonball Cut” that came to light sometime between late 1868 and 1877, when the Union Pacific Railroad cut through a ridge to relocate their main line track.  In 1868, the track had been laid temporarily to the west of Miser (now it is spelled Meiser) Creek; the new cut took a straighter course and saved a number of miles.  However, in the rush to meet up with the Central Pacific at Promontory, Utah, the UPRR postponed this construction until labor and machinery were available to cut through the ridge east of Rock River. 

Railroad workers were fascinated by the “cannonballs” they unearthed and probably took many as souvenirs.  However, they would have been useless for military or any other function compared to the metal used for actual cannonballs. The workmen who made Cannonball Cut had no idea of the possibility of a connection with dinosaurs because excavating the dinosaur fossils that are about 20 miles away at Como Bluff did not begin until 1878. (Those fossils were also first noticed by railroad workers.) Occasionally, an heir or amateur geologist will bring one of the cannonballs to the UW Geology Museum, disappointed to learn that they are not dinosaur eggs. 

Concretions such as the cannonballs are relatively common in many different rock formations. They may differ in shape from the one in the photo. Examples of irregular concretions mistaken for fossils are on exhibit in the UW Geological Museum. Others can be seen in the sandstone outcrops on each side of Highway 130 where it descends from the prairie to lower elevation several miles east of Centennial.

In 1900, the Union Pacific Railroad relocated the track in that area again to save about 12.5 miles.  They abandoned the town of Rock Creek and its station (a ghost town, all but obliterated by time and vandals now) as well as the old “Cannonball Cut” where these concretions are coming to the surface (see photo).  The ranchland has reverted to private ownership now and is not open to the public.  However, there are a number of these odd round “cannonballs” in private collections around the region, passed down through the generations of early amateur explorers looking for unique phenomena in the landscape of Albany County. 

By Larry Ostresh,

Caption:  A close-up of Cannonball Cut along an abandoned UPRR rail line in Albany County, with examples of the calcareous concretions that are commonly called “Cannonballs”.  The inset photo shows one that is about 6” in diameter.  These are not to be confused with geodes, which are formed in an entirely different manner. Nor are they dinosaur eggs or other fossilized remnants of the Jurassic era.  Courtesy photos by Larry Ostresh