The April 7 post about “Bowling Ball Ridge” along Fort Peck Lake got so much interest it begged for an explanation. (And frankly we were asked for one.)
These giant bowling balls are called concretions because the sediments were cemented by the mineral calcite or other binding agents. They may start when an animal dies in the bottom of a lake or sea which then releases calcium as it decays. Or there may be nothing in the middle because the particle dissolved. More and more sediment is cemented together over time until the calcite formation stops; all the cementing mineral is used. Consistent and even pressures allow the concretions to be perfectly round such as these balls. Some are more elliptical shaped, like eggs, though they are not fossil dinosaur eggs. They can be seen in all sizes around Fort Peck Lake, from marble size to VW Bug-size. Some are more rust colored due to a brown, iron-bearing carbonate mineral called siderite. Some are elongated, forming flattened or irregularly shaped concretions due to the constraining forces around them. The size of the concretion may be related to the amount of binding agent and the permeability of the sediments: the more permeable the surrounding material, the larger the concretion. Over time, these weather resistant concretions stay intact while the softer sediment around them erodes, leaving the concretion exposed, sometimes perched on a pedestal. Because perfectly round rocks are unusual in nature, these bowling balls are surprising. Remember to enjoy what you find with a photo rather than disturbing it. Let another person relish the discovery of these curious features.