I took the photos below during my visit to Theodore Roosevelt National Park (The Badlands) in North Dakota. The spherical boulders featured in the photos are a type geological formation called “cannonball concretions”. Concretions can range from fractions of an inch to more than 9 feet wide. They are formed when minerals are deposited around a core. Many concretions found in sedimentary rocks like sandstone or mudstone have a fossil at its center. These concretions form when carbon salts such as carbonate or bicarbonate leach out from decaying organic matter and react with calcium in the surrounding environment to produce calcium carbonate (calcite) which precipitates around the dead organism also acting like glue that traps in grains of sediment. Eventually, the layers of sediment around the concretion are compacted and cemented into rocks. The material of the concretion is harder and therefore more resistant to erosion than the surrounding rock. You can see some of the concretions in the photos below are still surrounded by the softer sedimentary rock that is slowly being eroded away.