Sandstone: Sandstone, a type of sedimentary rock, looks like sand frozen in place. When quartz, feldspar, and other silica-containing minerals and rocks break into fragments between 0.1 and 2 mm (0.004 to 0.08 inches) across, scientists call the pieces sand. In deserts, on beaches, and under bodies of water, layers of sand grains build up over thousands or millions of years, until the accumulated pressure from the weight of those layers compact the sand grains into solid rock, a process called lithification. The sand grains are commonly cemented together by fine-grained quartz and calcite. Sandstone makes up 10 to 20 percent of all sedimentary rocks on Earth because its ingredients are among the most widespread minerals. They are found worldwide and form under a wide range of depositional environments and conditions. Detailed examination of the mineral grains and rock fragments in sandstones is necessary to help geologists interpret the source rock and the environment in which the sandstone was deposited. For example, coarser sand grains in the rock could indicate that blowing wind or running water removed the smallest, finest granules before lithification took place.

Bivalves first made their appearance on earth about 500 million years ago during what is called the Cambrian Explosion when there was a great diversification of life. The earliest bivalves burrowed in soft sediments on the bottom of the ocean. They lived just under the surface, not deep down like some modern bivalves. Over time, bivalves evolved structures allowing them to bury deeper such as snorkel-like tubes (siphons) to get water from above; and also evolved modified gills for filter feeding. Bottom-dwelling (benthic) bivalves became fossilized when sediment hardened around them. Fossil evidence is a shell itself or just an impression of the shell left in the sediment (an ichnofossil). Bivalves have evolved into many, distinct forms over time, and survived through major extinction events to become a very diverse group of organisms. Most modern forms are benthic, but others attach themselves to hard surfaces or even swim around a bit (e.g., scallops).

Mollusks have inhabited the Earth for at least 540 million years, dating back to the early Cambrian period. The fossil record of mollusks consists mostly of shells. Rarely, it includes mouthparts (radulas) or the trapdoors (opercula) that cover shell openings. The rest of the body is soft and usually does not fossilize. Before the Cambrian, taxa such as Kimberella had many traits expected of early molluscs, but no shells or spines. Cambrian, worm-like animals with spiny scales, such as Wiwaxia, had mollusk-like mouthparts. The earliest agreed-upon mollusk looked like a snail with a single, cap-like shell that curved at the tip. More than 90,000 living species of mollusk have been described, plus another 70,000 species known only from fossils. Because many mollusks live in remote places such as in the bottom sediments of deep ocean, and new species are being discovered at a rapid rate, scientists think that many more species remain to be described.