Pisoliths

Pisoliths are a type of concretion, which are mineral masses that form within sedimentary rocks. They are characterized by their spherical or subspherical shape and their internal structure, which resembles that of a pea or a tiny egg. The name "pisolith" comes from the Greek words "piso" meaning pea and "lithos" meaning stone.

Internally, a pisolith typically consists of concentric layers of mineral material surrounding a nucleus. This nucleus can be a grain of sand, a fossil fragment, or even a void. The layers are often composed of calcium carbonate (calcite) or iron oxides. The formation process involves the gradual precipitation of minerals from groundwater that percolates through the surrounding rock. Dissolved minerals in the water deposit themselves around the nucleus, building up the concentric layers over time.

Pisoliths are found in a variety of sedimentary environments, including limestones, sandstones, and soils. They can range in size from a few millimeters to several centimeters in diameter. While often considered curiosities, their presence can provide clues about the geological history and groundwater conditions of the area where they are found. They are not to be confused with ooids, which are similar in structure but form in marine environments and are generally smaller. Pisoliths are more commonly associated with terrestrial or freshwater depositional settings.