Paleontology and geology
The Precambrian: Precambrian rocks occur only in Delaware’s Piedmont region at the northernmost end of the state. These metamorphosed rocks preserve the history of an approximately one-billion-year-old mountain-building event called the Grenville Orogeny.
The Paleozoic: The Paleozoic rocks of Delaware are metamorphic and so do not contain fossils. Metamorphism took place during a series of tectonic events that built the Appalachian Mountains.
The Mesozoic: There are no Jurassic or Triassic rocks in Delaware. The Cretaceous sediments record a change from terrestrial environments in the Early Cretaceous to shallow marine environments in the Late Cretaceous. Plant remains are common in the terrestrial sediments, while the marine sediments contain fossils of invertebrates as well as those of marine reptiles, dinosaurs, and pterosaurs.
The Cenozoic: A sea covered Delaware during most of the Cenozoic, and sea level fluctuated throughout this time interval. Major rises and falls of the sea during the later part of the Cenozoic left a record of alternating sandy nearshore and muddy offshore environments and, ultimately, bay environments. A diverse fossil record includes the remains of ancient relatives of horses, rhinoceroses, porpoises, whales, seals, manatees, bats, beavers, dogs, birds, snakes, fish, snails, and molluscs. Most of the state is covered by a veneer of sandy Quaternary sediments deposited by rivers formed from melting glaciers. Fossils are rare in these deposits.