A thin calcareous sandstone is overlain by fine sandy marl and sandstone with calcareous concretions. Calcite concretions in clay are called septarian concretions. Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix. Sometimes ironstone hosts concretions or opal gems. Typical examples include&colon; bedform armor, clasts, concretions, drift, dust, rocky fragments, and sand. The shale is overlain by the thicker Valtos Sandstone which contains concretions. Septarian concretions or septarian nodules, are concretions containing angular cavities or cracks, called "septaria". Most of these concretions are oblate spheroids. Other "pop rocks" are small polycuboidal pyrite concretions, which are as much as in diameter. Analysis indicates that these spherules are apparently concretions formed from a water solution. The rover indeed found layered rocks and marble- or blueberry-like hematite concretions. It is well known for preserving fossils three-dimensionally in limestone concretions, including many pterosaur fossils. Cannonball concretions are large spherical concretions, which resemble cannonballs. These are 1 friable soil with high content of secondary concretions of sesquioxide clays. Many gem-quality ammonites are found within siderite concretions.