: Because concretions are harder than the surrounding rock, they often remain after the softer host rock has weathered away, creating dramatic landscape features. Common Types and Compositions

: Groundwater rich in minerals (like calcite or iron) reaches a point of saturation and begins to solidify.

: These minerals fill the pore spaces between sediment grains, acting as a "natural glue" that binds the mass into a hard rock.

The composition of a concretion usually depends on the mineral present as a minor component of the host rock. Main Mineral Key Feature Calcite, Siderite Sandstone, Shale Often spherical; most common type. Siliceous Chert, Flint, Jasper Extremely hard and glassy texture. Ironstone Hematite, Goethite Black Shale High iron content; can look like rusted metal. Septarian Calcite, Siderite Clay, Mudstone Features internal radial cracks or "septaria". Notable Examples