By Frank Bliss
Folks are always telling me they have found some really cool “dinosaur eggs”. I tell them they are probably not dinosaur eggs. These rounded, spherical or elongated rocks that are often rusty red or yellow in color often fall out of sandstone outcrops. These objects also often take on the persona of a cannon ball to the non-geologists out there. Heck, they are heavy, round, redish and look like the right caliber for an 8 pounder. There is a simple (sort of) explanation.
These fairly common and certainly very interesting objects are called Concretions. They are not man made and are not reptilian in origin either. They are, in fact, naturally made by a somewhat complex series of events concluded with a simple process of cementation. The following process is necessary for their formation.
Please bear with me here and use your minds eye. Try to think of a thick bed of uniform sandstone as a 3 dimensional fabric (like a finely woven cloth but in three not two dimensions). This 3 dimensional fabric allows water to flow freely through the holes between the sand grains. A uniform fabric allows water to flow smoothly without obstruction unless a fragment of something other than sand (fossil, twig, root, rock, etc) disrupts a smooth passage of water. Moving water (like in pipes in your house) creates an electrical field which in a uniform flow is (guess what) uniform. Any disturbance in the water flow creates a disturbance in the electrical field which causes an “electric potential gradient”. (don’t let that term scare you!) An “electric potential gradient” means that the disturbed area might be more electrically positive (or negative) than the rest of the sand body. This causes charged dissolved minerals in the water (ions) to be attracted to that area. These minerals that are attracted to that charged area literally accumulate there and cement the sand grains together like concrete (remember the term concretion!).
In other words, minerals in water flowing through the sand selectively cements together sand grains around a nucleus. This process generally leads to very spherical, often concentric growths. Adjacent spheres may even grow together or an odd shaped nucleus can give very unusual final forms. I think they are neat enough to take home when they are small enough to carry. I have seen them the size of a small car and often they weigh many tons.
Over the years, I have collected a nice assortment of sizes, shapes, and types of cemented concretions which mostly reside in my gardens. The composition of the cement varies greatly and includes: pyrite, marcasite, hematite, calcite, halite, dolomite and various silica minerals. The iron cements are the most common around here though. Some rare concretions may even have very nice fossils (nucleous) within. Fortunately for us, Powder River County has more than it’s fair share of these nifty natural objects. Now you know they are not eggs or old cannon balls. I don’t know too much about cannon balls but I’ll cover dinosaur eggs in a later geologic column.