Photographs by Dan Phelps.
Mr. Phelps explains,
Gastroliths are usually defined as stones that were swallowed by herbivorous dinosaurs, or other animals, to aid in digesting food. Many types of vegetarian dinosaurs did not have grinding teeth and thus swallowed their food without chewing. This process is analogous to the tiny stones swallowed by some modern birds that are kept in a gizzard to aid in the digestion of food. Other animals, such as the marine reptiles known as plesiosaurs, swallowed stones for a different reason, namely to create ballast and thus be able to dive deeper.
Usually, gastroliths are highly polished stones made of very fine-grained chert, like this specimen. Sometimes they are also made of petrified wood and even, on occasion, igneous and metamorphic rocks. Unless a gastrolith is found within an articulated dinosaur skeleton, there is always a bit of justified skepticism about any well-polished rock identified as one. There is a possibility that this is only a rather well-polished extraformational pebble that was somehow deposited in fine-grained sediment in the Morrison/Cloverly, but this possibility doesn’t effect the point I will make below. The possible gastrolith illustrated is made out of chert and contains fossils of crinoids and other organisms. In any case, these rocks were already lithified before being deposited in younger rock formations. They contain fossils of life that existed before the Jurassic and in many cases before the Mesozoic. This is true of such pebbles whether they are true gastroliths or merely well-rounded, polished pebbles.
Most Young Earth Creationists (YEC's), such as those who built the Ark Park, claim gastroliths formed in one year during Noah’s Flood (usually claimed to have occurred in 2348 BCE). The sequence, precursors of the gastroliths formed from ancient sediment, then lithified, then re-eroded, then partly rounded, then swallowed by herbivorous dinosaurs, then further polished in the dinosaur’s digestive system, is totally inconsistent with YEC claims.
I thought of this long ago, and just discovered this well-done YouTube video by Erika Gutsick Gibbon (possibly a pseudonym) explaining why gastroliths are inconsistent with YEC.